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.
There are others, but this is all I have time for now.
Hope everyone has had a happy holiday!