1. NicheBot.comThis free keyword search tool would be worth paying (a reasonable fee) to access. Like other tools, it lists alternative keyword suggestions, but it also breaks things down by major search engines and keyword phrases. It only lists the past 100 days (free version) so you may wish to use this service in conjunction with KeywordDiscovery.com to get longer-range statistics.
NichBot was chosen as the top free resource because in addition to great keyword search features, it also offers (free) a thesaurus, keyword ranking, and keyword tracking tools.
Drawback: you must enter a "captcha" for each new search (image-based words and jumbled letters you enter to test whether you are human or a robot), but you can avoid this by registering for free.
SpellWeb is a very simple keyword phrase comparison tool. Enter two similar phrases and see which one is more popular. For example, enter “childrens specialty apparel” and “childrens specialty clothing” to see which phrase is more commonly searched.
This is a fast, free keyword tool that helps you create meta tags based on search popularity. The drawback is that it does not offer related keyword searches or suggestions. Pluses: it is fast because you do not have to type in cryptic code each time you compare something, you do not have to join anything to access the tool, and it is free.
UPDATE: In 2011, this domain stopped offering free keyword tools.
5. URL.comURL.com offers a unique SEO tool. Enter keyword phrase and see the top search engine returns from Google, Yahoo, and MSN. Using this information, you can visit competitor’s sites and review their meta data to optimize your own site, or use a keyword density comparison tool to speed things up.
While it is not a great tool for SEO, Google's AdWords tool may be useful in helping help you decide what key words to select, determine a reasonable price or bid, and which ad service to use.