good keyword tool also can be considered and its free http://www.goodkeywords.com/ secondly, i would recommend word tracker..it's been use in Hypervre case study if i'm not mistaken
I'm also using keyword tracker and yahoo overture as well but it seems that overture is frequent in errors.
Well I have recommend Google Adwords suggestion tool and inventory overture but in these tools you didn't get lot keywords but competitive once
I like Wordtracker, but the other free keyword suggestion tools mentioned are also good. I have tried most of them and if you aren't happy with the number of results you are getting with just one of them, I would suggest combining the results of two or three of the free services.