http://adwords.google.com/select/KeywordToolExternal
One of the best places to go for keyword research. Also, its free.
http://adlab.msn.com
Great resource provided by MSN.
SEO Book Keyword Suggestion Tool
http://tools.seobook.com/keyword-tools/seobook/
Great people, great tool. Use it often.
http://www.kwbrowse.com
Great tool that provides results from lateral markets.
http://www.quintura.com
Similar to kwbrose.com.
http://www.spacky.com
Allows easy downloading of keyword results.
http://www.google.com/insights/search/
Good Keywords – Free and Paid version
http://www.goodkeywords.com
Traffic Travis - Free and Paid Version
http://www.traffictravis.com
Basically the free version is almost as good as the paid.
Wordtracker.com – Free and Paid Version
http://www.wordtracker.com
One of the oldest keyword research tools around. Good tool, but not the best. Worth using from time to time, also Seomoz uses this tool as a touchstone for some of the tools there.
SpyFu.com - Free and Paid version
http://www.spyfu.com
Spyfu helps you steal your competitors keywords. Expensive though.
http://www.gorank.com/analyze.php
SEO tool that helps determine the relevancy of your web pages for keywords of your choice.
http://ci.trellian.com
This tool compiles and analyzes usage statistics. You are able monitor web sites to identify the traffic sources. You can find out which sites are responsible for sending traffic to their pages, including search engines and the search keywords used.