By far the best I have found is flight tracker, which also tracks domestic flights on a map of the U.S. in real time. This is done directly from FAA transponder radar data so it is independent of the airline. It's also updated very often.

Click on

http://www.trip.com/trs/trip/flightt...acker_home.xsl

If for some reason you are not on the flight tracker home page -- they seem to be able to make an ad come up many times -- click on "travel resources" and then "flight tracker."