Google released the Search Based Keyword Tool back in 2007, but it is never discussed!
Update 3/22/2011: Google has announced that they will soon be deactivating their SKtool, so use it while you can
What the Sktool will help you discover new keywords you maybe missing out on based upon the search query data from your website or your competitor’s websites.
An “Inside AdWords Crewmember” describes the Sktool on Google’s blog:
.. you know that Google search engine starts with searches conducted by users and helps them find relevant pages. But for keyword targeting, what you want is a tool that goes in the […]