Other tools
Overture
Also useful, Overture's search term suggestion tool, is free and much quicker to use than Wordtracker. It works in much the same way as Wordtracker but doesn't tell you how many websites are targeting each keyword phrase.
Google AdWords Keyword Suggestions tell you which keyword phrases are being targeted by other websites.
Guidebeam
Guidebeam is an interesting resource. Type in a phrase and it will suggest a large number of related searches. The numbers provided for each phrase are Guidebeam's estimation of how relevant that phrase is.
This article was written by Trenton Moss. He knows an awful lot about accessibility and the Disability Discrimination Act.