A flight simulator of sorts, created by superimposing an airplane onto a backdrop of Google Maps. You can turn, go up (not really -- the mapscale doesn't increase) or down (too low and you crash) or fire your spacebar gun at nothing in particular.
I have no idea where the terrain is, but I feel safe to say that it has tennis courts.
Update: I didn't at first notice it, but you can pick from London, New York, Paris, Tokyo and Washington, DC.