Here‘s an interesting site. It tracks all the passenger airplanes in the air in the USA at any one time. Quite what you can learn from this, aside from what planes are in the air in the USA at any one time, I’m not sure, but it surely has its uses, for more than planespotting.
It’s useful, for instance, if you are hoping to meet a plane, I guess. Or blow one up. I can’t quite work it out, but I think it tells you if a plane is running late, while it is still airborne.
I tried to find similar info for Europe, but all I could get to was cheap flight offers.
I got to this site via this guy, who I got to thanks to an email from Michael Jennings, pointing out this, but that’s another story. (In my opinion the iPod toilet roll holder, which I had already viewed before Michael clocked it for me, is not tasteless enough.)