One of my favorite tools as an affiliate is WordTracker. I’ve been a longtime subscriber, because it helps me to optimize my sites by identifying the most popular keywords for a given niche.
WordTracker is also really good if you’re running PPC search campaigns, as it helps to generate relevant keyword lists to bid on.
The downside is that it’s not cheap. However, they currently have a promo to get 15% off the regular pricing if you buy if by October 31, 2007.
To obtain this promotion, enter this code for 15% off an annual subscription: us-affiliate-annual-2007
If you want to give it a test drive, you can get a free trial of WordTracker.
They also have a good newsletter, articles, and case studies on the site. Visit WordTracker for more details.