Overture and Google Keyword Suggestion Tool

Search Engine Optimization Tools

Offers suggested monthly regional search volumes by market for Google, Yahoo!, and MSN.*
Links the search volumes to the related global search results.
Provides links to price estimate tools from Overture and Google AdWords. That Google AdWords tool shows the necissary bid to rank #1 for 85% of queries, and roughly how much traffic you could expect AdWords to send you based on that bid price.
Links to Google Trends, Google Suggest, Google Synonyms, Yahoo! Suggest, Nichebot and Wordtracker keyword research results.
Links to various vertical databases like Topix.net, Google Blogsearch, and Del.icio.us to let you know if people are talking about your topic and what types of resources they are referencing.

Want more free PPC or keyword research tips? Need more Keyword Research Data?

If you use pay per click marketing you are probably best off using Google AdWords or Yahoo! Search Marketing over the smaller PPC engines due to typically higher click quality and faster feedback loops.
Wordtracker offers a subscription based keyword research service and has a free keyword research guide [PDF].
Aaron Wall offers a free Google AdWords & Yahoo! Search Marketing Tips PDF and this free keyword research video on how to use this tool.
Dan Thies's SEO Research Labs offers professional keyword research services