Search engine optimization is critical for your site to be found in the search engines such as Google or Yahoo. These tools will help give your site the power it needs for the best positioning in search results.
Article Directories
EzineArticles 
EzineArticles.com is the most popular article submission directory ever. Millions of people a month visit this site looking for information and you can drive these visitors back to your website with the clickable links you have at the end of each article.
ArticlesBase
ArticlesBase is another strong article directory that gets tons of views and great targeted traffic back to your websites and sales letters.
Keyword Density Analyzers
Wordtracker
To determine the popularity for keywords use Wordtracker.
GRSoftware
Another density analyzer, this tool will help you find just the right amount of keywords in your website content to make the search engines love you.
Search Engine Information
Perry Marshall’s Google AdWords Course
Google AdWords is Google’s pay per click program. This book is a good training manual to learn how to use AdWords to your advantage.
Planet Ocean is the premier source for all the up-to-date SEO information and cutting edge topics to give your website the best chance of being #1 in the search results.
Search Engine Watch
This website periodical arms you with the software, web tools and information that you need to keep your website relevant and on top of the search results.
Web Thesaurus
Thesauraus.com
Just like a printed thesauraus, this tool will give you alternative sources of keywords that you may not have thought of. Great for website content and copy.
Keyword Research
Good Keywords
This tool will give you keyword suggestions to help you find the right copy and content for videos, articles and webpage optimization.
Google Alerts
Alerts to get daily emails with blog posts related to targeted keywords.
Word Tracker
This service gives you the keyword suggestions you need to create webpages and titles for articles and videos. You can even use this tool to think of business ideas and products. It has a free or paid version.
Ad Word Analyzer
Use this tool to find great keywords for your Google AdWord campaigns. Find how many people are really looking for the words you are thinking of using.
Dictionary.com
An online version of a printed dictionary, it also gives you great alternative suggestions.