I love Flight Radar. You need a decent amount of memory to be able to display it properly though.
Flight radar shows you all the planes in the air meaning that if you know someone flying you can watch their plane leave, travel and then land. You can see the altitude and location of planes and it's quite interesting when there are conditions such as snow as you can see the planes circling round waiting for airports to open.
Okay, it may seem sad to some of you but it is worth a look, believe me!