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. http://freekeywords.wordtracker.com/
It’s not as robust as the full version but it will give you some good keywords and traffic estimations.

3. https://adwords.google.com/select/KeywordToolExternal
Google’s free keyword analysis tool, often used in conjunction with adwords.

Yahoo used to have a free tool with their acquisition of Overture but they have since taken it down.  We want it back!  I’ll keep you updated when/if one becomes available.