Google just released the AdWords Keyword Suggestion Tool [users must be logged for the link to work].
The Google Adwords Keyword Suggestion Tool features:
Keyword list sorting. Sort the results of your keyword search by popularity, performance history within the AdWords system, cost, and predicted ad position. Easy keyword manipulation. Select a few keywords here and there, or add them all at once. Keywords already present in your Ad Group will be marked so that you don’t have to worry about them. You can also download your keyword list as a .csv file. Search for keywords in three ways. Use keywords you enter, your existing high clickthrough rate keywords, or any webpage URL for your search. You can also expand your keyword search even further to include pages linked to from the original URL. (Note: Site-related keyword searching is currently only available for English language users.) More keyword results based on regularly updated statistics. Our advanced search engine technology allows us to provide you with the latest information on potential keywords for your campaigns.
You can also generate additional keywords with the tool by extracting page content from your site. They offer instructions on how to use the keyword suggestion tool.