Competitive Intelligence on the Web

This site purports to tell you a lot about your competitor's site traffic.  Not sure how they can do this accurately but interesting if it's true. 

Trellian compiles and analyzes internet usage statistics to create a powerful Competitive Intelligence tool that no business should be without.

Competitive Intelligence provides the means to monitor your competitor's web sites to identify their major traffic sources. You can find out which sites are responsible for sending traffic to their pages, including search engines and the search keywords used.