I'm a big fan of the Cerebro tool which helps me optimize my PPC campaigns by honing in on the best keywords to use. The ability to see both search traffic and level of competition for each keyword is very useful. I also like the Magnet and Keyword Tracker, and I use Scribbles to make my product listings. I use X-Ray almost daily to check sales data for product research. I'm looking forward to trying out their new Follow-Up too for my email campaigns. I really appreciate the multi-pronged approach of this application. This is an all around excellent resource.