FlightStats mashes up the real-time flight information with Google Maps. Like other flight trackers, it shows scheduled departure and landing times, as well as actual take-off and projected landing. It will also send you e-mail or SMS alerts if the flight status changes, which is highly useful if your carrier or Web ticketing service doesn't offer such a feature.
FlightStats also has a very good mobile-friendly site and applications for certain handsets. It's the one you'll use when you're hanging around in the airport wondering just how far away a flight you're waiting for is.