WordTracker (Free trial) - The industry standard in choosing effective Keywords. Provides live keyword search results derived from meta crawler databases. Also runs and lists website competition, search counts and 24 hours predictions for each key phrase listed.

Priority Submit WordTracker Alternative Keyword Research Tool (Subscription Service) - Utilizing a database comprised of over 9 Billion searches. Priority Submits keyword research tool offers everything WordTracker has to offer as well as some new statistics. Such as seasonal trends and a keyword effectiveness indicator. If there ever was an alternative to WordTracker, this would be it.

Overture Search Term Suggestion Tool (Free) - Quick and easy to use tool for viewing individual search terms and how often that term was searched for in the past month. Large count numbers are skewed by Overture basing their results on more "searches" than the word tracker results as well as Overture combining both singular and plural versions of the term in their results. Still a good tool for quick estimates however not as accurate as WordTracker.

123 Promotions Overture/WordTracker Keyword Research Tool (Free) - An excellent free tool based on Overture and WordTracker results. This tool also provides search results from the previous month as well as hourly, daily and weekly projections.

Webmaster Toolkit Related Keyword Suggestion Tool (Free) - An easy to use & free tool for researching related search terms from whichever search engine you choose.

Digital Points Keyword Suggestion Tool (Free) - Combining Word tracker and Overtures keyword results side by side for individual key phrases. Great little tool and it's free to use.

Global Promoter (Free) - Provides single search term and multiple search term results derived from the Overture network. Including - Yahoo, Lycos, MSN, InfoSpace, CNET, AltaVista. Global Promoter also offers a variety of other tools related to search engine marketing. Click here to view their entire compliment of free tools.

Google AdWords Keyword Tool (Free) - This tool was built for Google AdWords and is designed to provide the AdWord users with a large variety of derivatives, and variations of different yet related keyword suggestions.

AltaVista Related Keywords (Free) - Run a search in AltaVista for one of you keywords and AltaVista will return a SERP along with a small grouping of related search terms in the upper right hand corner of the screen.