Here’s an interesting Keyword Research Tool using Overture and Wordtracker data. It goes a step further than some of the free tools and offers suggestions and a sort of score for how difficult it might be to rank well for a particular keyword phrase.
Another similar free keyword suggestion tool using both Wordtracker and Overture data is offered at Digital Point.
Google suggest can be useful for Google-specifc keyword reseach.
DaveN has made a cool tool that uses Google Suggest data so you can scrape more than the default 10 search term suggestions.
Of course there’s the Overture Keyword Selector, Wordtracker and the Google AdWords Keyword Sandbox.
There are others, but this is all I have time for now.
Hope everyone has had a happy holiday!