Except that it's not really real-time. As far as I'm aware, all of the sites like this are delayed by about 20 minutes from real-time.

Another impressive option is available using Google Earth, though you have to link to it via another flight-tracking site. If I recall correctly, you do a Google search for the flight number (e.g., "DL 160") and the first option Google returns is to track the flight using one of three different sites (but only if the flight is in the air). You have to select the "fboweb" option, and then on the fboweb page look for the link to the Google Earth tracking option. Then you can track the flight in 3D. That's right, you can lower the viewing angle and see the plane symbol and its path in the air above you.