The google keyword tool is the best tool here is the link https://adwords.google.com/select/KeywordToolExternal
that is a good tool. Keyword Elite is what i use and it generates up to 10000 keywords which is enough!
- Overture - Google Adwords Keyword Tools - Wordtracker ... and many others. There're even tools that offer all of the above (plus their own keyword research tools) in one place, making it much easier to manage keywords from different sources like, say, Rank Tracker
I use the following: Wordze.com Keyworddiscovery.com Microsoft Ad-Intelligence Plugin for excel 2007 (this thing rocks!)