Rankforest keeps track of the sales rank metric made available by Amazon.com through their generous and extremely useful web APIs. It also monitors the sales ranks published by Barnes and Noble. Frequently these metrics are of great use to authors, publishers, and analysts. While it's quite difficult to relate the sales rank information directly to sales, it is a useful measure of popularity.
Rankforest began tracking some books and CD's back in 2005. Since then many authors have added their books and used the system to monitor the effectiveness of promotional campaigns and to satisfy a curiosity of market reception.
As of December 2007, nearly 2.5 million individual rankings had been collected for 2,800 items submitted by 1,500 users.
Rankforest is user-driven. Many of the sites features come as a result of feedback from individual users. Please don't hesitate to contact me if there's something you want to see.
For information about the site from others, please see Weber Books, Kevin Kelly's Cool Tools, and the Amazon Web Services Blog.