The best in my opinion is "Keycompete.com". Which is in my sig line. I have tried wordze.com, but it seems to get a lot of unorganized topics and keywords. If you want free that try Google keyword tool, very good tool. Don't forget to get Adwords Editor to group those keywords in like terms and make adgroups on the fly.
Keyword Companion is also a great tool you can use it to organize your AdGroups too Just look for it on Google!
It is a very good keyword tools and you can try its feature keyword for free. http://freekeywords.wordtracker.com/
ronaldec7 is the only one correct in this thread. KeyCompete is not one, but THE best ADWORDS keyword tool. Nothing compares and that I can guarantee without a shadow of a doubt. Nichebot is also good, but not nearly as powerful for adwords keywords like Keycompete. When it comes to seo stuff, then nichebot or themezoom is the way to go.
If I read this post a month ago I would say it ain't that great, however as my experience and knowledge grow I would say it's one of the best keyword tool. Why? since Google accommodate more than 50% searched per day than other search engines combined, it surely has the biggest keywords database in the world of search engine, along with its traffic and ad position estimator tools I'd say it's the best for the moment.
google keywordtool is good but its best to run those key words you get through wordtracker.com and one other key word site and then youll end up with the very best long tailed key words to include.
The thing is...there isn't enough data for complete analysis like you would get with KeyCompete. I mean damn...you can spy on your competitors keywords with keycompete. That data alone is worth the price. The google keyword tool is a bit vague while keycompete uses a vast amount of premium resources including the Google keyword tool to gather very good data for analysis.
I just found a blog covering "why should you check your keyword research with Google?" SOL Hope it helps.