Although I prefer to use the overture keyword tool to perform keyword discovery and research, it can be useful to have other resources to find the value of different keywords. Listed below are a few other sites that will help you get a general idea of keyword rankings.

http://www.goodkeywords.com/
http://www.digitalpoint.com/tools/keywords/
http://www.wordtracker.com/free-trial.html
http://www.adwordanalyzer.com/
http://www.iwebtool.com/keyword_lookup/
http://www.wordtracker.com
http://www.keywordelite.com
http://www.rankalert.net/keyword-popularity.php

Most of these other keyword tools dont have as large a sample to draw their data, but they can provide you with some interesting data that allows you to compare them to overture rankings and give you a more well-rounded view on your selected keywords.