Keyword density and prominence tool.
http://www.ranks.nl/cgi-bin/ranksnl/spider/spider.cgi?lang
This is a handy little tool will show you the results of your query from both Wordtracker and Overture for determining which phrases are searched most often.
http://www.digitalpoint.com/tools/suggestion/
Not sure what search terms to bid on? Find related terms.
http://inventory.overture.com/d/searchinventory/suggestion/
Another tool for reporting keyword density.
http://www.searchengineworld.com/cgi-bin/kwda.cgi
Type in a search term and they will show you the Max Bids and listings for that term
http://uv.bidtool.overture.com/d/search/tools/bidtool/
AdWords Competitor Research – find out what terms the competition bid on.
http://www.googspy.com/
Comprehensive directory of the cheapest keyword phrases for use with Google Adwords
http://www.cheapkeywords.com/