Look at turbosquid (www.turbosquid.com I think).

I used to own Disney Animation Studio, and really liked the way it handled multiple drawings so that you could "see through" and flip them as you worked. Geez - that must have been 10 years ago? Never got very far with it, though (bouncing ball, that sort of thing). I don't even remember what OS I was running at the time.

Attached Link: http://www.maxon.de

I'll throw a pitch in here for Maxon Cinema 4D R8. It is legendary for its stability, and the interface is, for me at least, very intuitive. R8 adds some pretty incredible stuff to an already great package. You can download the demo version the link provided.