FlightAware.com

FlightAware.com is a new online US flight tracker. Here's the blurb:

Founded in March of 2005, FlightAware is the first company to offer free flight tracking for both private and commercial air traffic in the United States. FlightAware's proprietory flight arrival time algorithms combined with our powerful, intuitive, and reliable web-based interface yield the most capable and useful flight tracking application on the Internet.

It does seem to provide a wealth of information about each flight and aircraft. The 24-hour animation of US flights is pretty cool.