Keyword Research Tools

Wordtracker
This tool allows user to find other relevant keywords, and assess frequency of use in search engine queries - a must for analyzing and targeting your keywords.
http://www.wordtracker.com

Overture Search Term Suggestion Tool
Another keyword research tool that evaluates the frequency of specified keyword use in Overture search engine queries. This tool is free of charge, but unlike Wordtracker it only allows to look up one term at a time.
http://www.overture.com

Google Keyword Suggestion Tool
This keyword tool lists related terms to the one you indicate. Primarily used for your Google AdWords campaign, but can be useful during keyword research as well.
http://adwords.google.com