I’m just using one library for my time-travelling 10-second game: Box2D, the Actionscript version. I had not used Box2D before this weekend, and I must say I’m pleased and impressed. It’s not vastly dissimilar to Chipmunk, which I’m more familiar with. I prefer some things about Chipmunk, but on the whole I find the object oriented interface to Box2D more to my liking. If I were working in C++ I’d compare the two based primarily on performance, but with a Flash game performance is never a huge ambition. If it’s good enough, it’s good enough. So the interface, the ease of learning—these things start to matter a lot. And Box2D has served very well in that regard.