Free Keyword Research Tools :

Overture Inventory- currently a Yahoo! Search Marketing product - generates a list of related searches that include your term and also shows how many times the term was searched for during the last month.

Related Pages- generates a list of possible keyword combinations based on lists of keywords that you provide

Google's Keyword Tool- generates potential keywords for your ad campaign and reports their Google statistics, including search performance and seasonal trends

Google Suggest- as you type, Google will offer suggestions

SEO Book Keyword Suggestion Tool- shows top keyword phrases from Overture; links to other keyword research tools and links at various vertical databases, blog searches and tagging systems to show you top results from other information sources

Digital Point Keyword Research Tool- shows you the results of your query from both Wordtracker and Overture for determining which phrases are searched for most often

Good Keywords- a free Windows software for finding the perfect set of keywords for your web pages

(Found these in a email, use on your own risk !)