Google AdWords Keyword Tool: Enter a search term or terms, and you’ll be shown other keywords related to that term. You can then sort keywords in terms of search volume popularity.
Google Trends: Got some terms in mind and wondering how they’ve been trending, such as becoming more or less popular? Google Trends provides a way to go back in time and chart keywords, based on the accumulated search data that Google sees. Also see Google Meme: Hot Trends Added To Google Trends, which explains more about the day-to-day reporting you can get from Google Trends.
Wordtracker Free Keyword Suggestion Tool: Enter a term and get a list of 100 terms related to (and including it), ranked by daily popularity. Data is based on Wordtracker’s search term data collected from the Dogpile and Metacrawler meta search engines. Wordtracker also offers a much more robust fee-based keyword research tool.
Trellian Free Search Term Suggestion Tool: As with Worktracker above, enter a term and get back a list of 100 terms related to and including it. They’ll be ranked by how often they’re searched for each year, rather than each day. Data comes from a variety of search engines. Trellian Keyword Discovery is fee-based tool hitting the same database and providing more features.
SEO Book Keyword Suggestion Tool: Taps into the Yahoo tool above and provides links and information to many of the other tools also listed above, so that you can get more information directly from them. A variety of other keyword tools are also offered.