Keyword Discovery and Wordtracker are great… if you want to pay for your resources.  But, if you’re on a small budget, you may find these tools suitable for your needs:
1. http://www.digitalpoint.com/tools/suggestion/
This tool will show you the results of your query from both Wordtracker and Yahoo! Search Marketing (formerly Overture) for determining which phrases are searched most often.
2. [...]