1. Web Developer Toolbar

The Web Developer extension adds a menu and a toolbar to the browser with various web developer tools. It is designed for Firefox, Flock and Seamonkey, and will run on any platform that these browsers support including Windows, Mac OS X and Linux.

2. SEO for FireFox

Pulls useful market research data right into Google’s and Yahoo!’s search results, including the following data near each search result.

Google PageRank, Age, Links (Yahoo! Linkdomain), .edu Link (Yahoo! .edu linkdomain), .edu Page Link (Yahoo! .edu link), .gov link (Yahoo! .gov link), Page Links (Yahoo! link), del.icio.us, Technorati, Alexa, Cached, Dmoz, Bloglines, dir.yahoo.com, WhoIs

3. User Agent Switcher

The User Agent Switcher extension adds a menu and a toolbar button to switch the user agent of the browser.  It lets you switch / view the current page as in a different browser (IE, Opera, NetScape).

4. Google Search Enhancer

This addon enhances your Google search by giving you more options like search by date (past 24 hours, by week, by month etc), filter by language, search for any file (mp3, doc etc). Pretty effective if you want to save time filtering through results manually.

5. Search Status

Display the Google PageRank, Alexa rank, Compete ranking and SEOmoz Linkscape mozRank anywhere in your browser, along with fast keyword density analyser, keyword/nofollow highlighting, backward/related links, Alexa info and more.

6. Google Global Search Plugin

Google Global is an unobtrusive Firefox extension that allows you to see what the Google search results that you are viewing look like from different geographical locations. The easiest way to use Google Global is to simply do a search for something on Google. When you see you search results page, simply right click anywhere on the search results page, select “Search Google Global” and select what geographic location you would like to see the results from. It is as simple as that.

7. K Gen – Keyword Research Firefox plugin

KGen (Keyword Generator) is an extension that allows you to see what keywords are strong on visited web pages. Than, you can retrieve them for social sharing (tag filling) or webmastering/SEO. KGen retrieves every words of a page and lets you know how many time they appears on her and their weight in the page (a word in the title of a page has a stronger weight than a word in a paragraph). So, you can easily know how to optimize your pages, know what’s the secret of your competitors or simply know what’s the main subject of a webpage.

8. yExplore – Yahoo Site Explorer

yExplore adds quick access to search engine & social media information. Just right click, select yExplore and then one of the shortcut links to access information about the site you are viewing. yExplore 2 give quick and easy access to the following information:

Yahoo Site Explorer Backlinks & Inindex, Google Backlinks, Inindex, Cache, Archive.org, Bing Inindex, Delicious saves for a domain, Highlighting of NoFollow links,  Check to see if the page you are on is in StumbleUpon,  View the Robots.txt file,  Spell check the current page,  Edit the current page,  Text only view,  Whois Info

9. FireShot Screen capture

FireShot is a Firefox extension that creates screenshots of web pages (entirely or just visible part). Unlike other extensions, this plugin provides a set of editing and annotation tools, which let users quickly modify web captures and insert text annotations and graphical annotations. Such functionality will be especially useful for web designers, testers and content reviewers.
The captures can be:  uploaded to FREE public screenshot hosting,  saved to disk (PNG, GIF, JPEG, BMP),  printed,  copied to clipboard,  e-mailed,  sent to configurable external editor for further processing.

10. ColorZilla – Pick Color from webpage elements

Advanced Eyedropper, ColorPicker, Page Zoomer and other colorful goodies.With ColorZilla you can get a color reading from any point in your browser, quickly adjust this color and paste it into another program. You can Zoom the page you are viewing and measure distances between any two points on the page. The built-in palette browser allows choosing colors from pre-defined color sets and saving the most used colors in custom palettes. DOM spying features allow getting various information about DOM elements quickly and easily.

11. SearchPreview

Inserts preview images (thumbnails) and popularity ranks of web sites into the Google, Yahoo and Bing search results pages. The SearchPreview thumbnail system currently consists of 9 dedicated servers and generates over 15 TB of traffic per month. For a very few search terms, this version will insert sponsored links into the google.de and google.com search results (labeled as ‘SearchPreview Sponsored Links’). If you don’t like the sponsored links, you can disable them in the SearchPreview options dialog.

12. Alexa Sparky

Sparky is Alexa Internet’s free Firefox plugin. Sparky accompanies you as you surf, providing useful information in your status bar about the sites you visit without interrupting your browsing.  Discover website traffic trends, Get detailed traffic information including Reach and Rank , surf more efficiently with Related Links for each page, Gracefully navigate past dead end error pages.

13. Search Cloudlet

Search Cloudlet inserts smart tag clouds into Google / Yahoo / Twitter interface to help you search faster. This free productivity-boosting add-on was featured at LifeHacker, PC World, Website Magazine, etc. Install it now to add magic to your search. One of the most useful features provided by Search Cloudlet is Tag Cloud for search results. Now there is no need to look through results list to understand what it’s all about. Tag Cloud will show you all the keywords relevant to your search results. And you can easily narrow your search by clicking keywords on the cloud and adding/removing them to/from your query.

14. Show IP

Show the IP address(es) of the current page in the status bar. It also allows querying custom services by IP (right mouse button) and hostname (left mouse button), like whois, netcraft. Additionally you can copy the IP address to the clipboard. This extension was formerly known as ipv6ident.

15. Domain Details

Displays Server Type, Headers, IP Address, Location Flag, and links to Whois Reports. Shows links to check server status and cache when page fails to load. View the current site’s server software and headers, IP address, location flag, and have quick links to Whois and other domain information. When a page fails to load, links are shown to check cache and server status.

Drupal, Joomla and Wordpress are very popular and powerful content management systems. They are all written in PHP, support the MySQL DBMS and are built on a modular architecture that allows for extending them with additional functionality.

There are hundreds of free add-ons (modules, components, plugins) available for each of these three systems. Some of them are especially useful when it comes to optimizing your website for search engines.

Below you find a selection of such SEO related add-ons for Drupal, Joomla and Wordpress listed in alphabetical order, including tools for search engine friendly URLs, meta tag generation, social bookmarking services, performance tuning, user tracking and statistical analysis. Add-ons related to search engine friendly URLs usually require mod_rewrite to be enabled on your Apache web server.

Have fun playing around with these tools and bear in mind that none of them is a replacement for the most powerful SEO technique, which is creating valuable content.

Drupal Modules

1. Seo Friend is meant to be used along side existing Drupal SEO modules to make them more effective. This module does not replace functionality available in the SEO Checklist and SEO Compliance Checker modules.
2. Seo Compliance Checker checks node content on search engine optimization upon its creation or modification. Whenever a publisher creates or modifies a node, the module performs a set of checks and gives the user a feedback on the compliance of the rules.
3. Seo Checklist provides a checklist of good Drupal SEO (Search Engine Optimization) best practices. Maximize the presence of your Drupal website in the major search engines like Google, Yahoo, Bing, etc.
4. Block Cache creates cached versions of each block displayed on your website. For each cached block only one SQL query is executed thus reducing server load resulting in better performance and faster delivery of content.
5. Find URL Alias is a utility module that lets you search for particular URL aliases. This module requires the core path module which enables search engine friendly URLs for your site. Find URL Alias is especially useful when there are many aliases stored in your database. But keep in mind that changing URLs is something you should avoid.
6. Google Analytics integrates Google’s powerful web statistics tracking system with your website. The module enables you to selectively track users by role.
7. Meta tags allows you to define site wide meta tags and specific tags for each piece of content (node). If you use the core taxonomy module (you really should do) meta tags can be assigned automatically by using the terms (tags) you use to categorize your content.
8. Pathauto automatically generates path aliases based on the modules settings. A very powerful module that lets you define different URL patterns based on content types. In the latest version also allows to filter common words. Requires the core path module.
9. RobotsTxt is useful if you run multiple Drupal sites from a single code base and want to have different robots.txt files for each of them.
10. Service links automatically adds links to social bookmarking and blog search services to your content. You can select which services you want to link to, restrict the display based on content (node) types and whether to display links in teaser and/or full page view.
11. URLify automatically generates the path alias for a piece of content based on its title using JavaScript. Requires the core path module. I lightweight alternative to pathauto.
12. XML Sitemap generates an XML sitemap which complies with the sitemaps.org specification. The relative priority of each piece of content is calculated based on content type, number of comments, and promotion to front page. The values of each of these factors can be set in the admin section of the module.

Joomla Modules, Components, Plugins

1. Content Search Seo Plugin is based on the official search content plugin, and add some seo features. It will generate page title, meta description and meta keywords dynamicly according to the search keywords and search results.
2. SEO Canonicalisation Plugin enables site ‘canonicalisation’, which means that when a user hits your site from another domain than your preferred.
3. SeoSimple simply takes the starting chunk of text from the page’s content and applies that as the value for the meta description tag in the page’s head.
4. Seo Generator automatically generates keywords and description by pulling text from the title and/or the content to help with SEO.
5. Seo Keyword link is very useful to improve the internal pages SEO inside your site. you can define the limit of links to each keyword , NoFollow or DoFollow and open link in same window or new window.
6. Advanced SEF Bot for Joomla 1.1.x is a plugin that enables search engine friendly URLs. Since Joomla’s standard URLs are not really meaningful this one is a must.
7. Dynamic gSitemap is a PHP script that dynamically creates an XML sitemap of your site when GoogleBot visits it.
8. JoomSEO is a plugin that dynamically creates meta tags, changes the title tag on the fly, adds heading tags to content titles and more. Some of the configurable features include: show, hide or override keywords, site name, and content title, adjust element order in the title and heading tag selection.
9. LinX is a link exchange component that lets you manage a reciprocal link directory. Links that are submitted to your site are automatically checked and only added if there is a link back to your site. A large number of links to your website of course has an effect on the PageRank but what really matters are links from quality and trusted websites that have a similar target audience as your site.
10. MetaTags NX creates meta description and keyword tags for your content on the fly. Keywords are generated based on their frequency in the content and stopwords can be excluded.
11. Redirect component lets you redirect old urls to new ones and set their status codes. Remember, changing URLs often is a bad idea.
12. Website Validators Tool contains links to validation and site information services that help you make your website more standards compliant, see how you rank who links to you and more.

Wordpress Plugins

1. All in One SEO Pack Optimizes your Wordpress blog for Search Engines.
2. Seo Title Tag optimizes the title tags across your WordPress-powered blog or website.  Not just your posts, not just your home page, but any and every title tag on your site
3. Seo Slugs removes common words like ‘a’, ‘the’, ‘in’ from post slugs to improve search engine optimization.
4. HeadSpace2 SEO is an all-in-one meta-data manager that allows you to fine-tune the SEO potential of your site.
5. Seo Friendly Images is a Wordpress optimization plugin which automatically updates all images with proper ALT and TITLE attributes. If your images do not have ALT and TITLE already set, SEO Friendly Images will add them according the options you set. Additionally this makes the post W3C/xHTML valid as well.
6. Seo Post Link makes your post link short and SEO friendly. It removes common words that are unnecessary for search engine optimization of your blog post link.
7. Seo Smart Links provides automatic SEO benefits for your site in addition to custom keyword lists, nofollow and much more.
8. Platinum Seo Pack Optimizes your Wordpress blog for Search Engines (Search Engine Optimization).
9. Simple Submit Seo / Social Bookmarking Plugin is for adding submission links for Digg, Delicious, Buzz, and Stumble to pages and posts.
10. Automatic Seo Links Forget to put manually your links, just choose a word and a URL and this plugin will replace all matches in the posts of your blog.
11. Seo for Paged Comments reduces SEO problems when using WordPress’s paged comments.
12. Seo No Duplicate helps you easily tell the search engine bots the preferred version of a page by specifying the canonical properly within your head tag.
13. 404 Seo Plugin gives you a customized, smart ‘Page Not Found(404)’ error message. It will automatically display links to relevant pages on your site, based on the words in the URL that was not found.
14. Add to Any adds links to a large number of social bookmarking sites to your posts.
15. GeneralStats is a statistics components that counts the number of users, categories, posts, comments, pages, words in posts, words in comments and words in pages. Useful for doing keyword research.
16. Google Sitemap Generator creates an XML sitemap of your website. In the current version homepage, posts, static pages, categories and archives are supported. Priority is automatically assigned based on the number of comments.
17. Gregarious is a social bookmarking plugin for Digg, Reddit and Feedburner with update checks via AJAX.
18. Popularity Contest is a counter for posts, categories, archive views, comments, trackbacks, etc. to determine the most popular pages of your site.
19. Technorati Tagging Plugin adds Techorati tags to your posts and enables you to display a tag cloud.
20. WP-Cache is a page caching system to improve your websites performance. Cached pages are stored as static files, reducing server load thus making your site faster and more responsive.
21. X-Valid attempts to convert posts and comments to valid XHTML. Read more on the benefits of Web Standards Compliance.
22. Search Engine query is trying to reduce the bounce rate of your blog and provide the visitor a better navigation experience.

If you know other free SEO related add-ons for Drupal, Joomla and Wordpress that should be mentioned here simply post links to them in your comments. I’d like to read about your experiences using these add-ons.

SMF:

Seo4SMF – http://www.webmasterstalks.com/tpmod.html;dl=item49

phpBB:

phpbb-seo (SEO mods, and a pre-modded phpbb) – http://www.phpbb-seo.com/

MyBB:

MyBB SEO – http://mods.mybboard.net/view/mybb-seo

vBulletin:

vBSEO – http://www.vbseo.com/

Invision Power Board:

MinervaSEO – http://minervaseo.com/