OEcake!
its a physics engine a friend showed me, theres so much to have fun with, its hard to know where to start!
the official site stopped giving out downloads, but a friend is hosting a DL on his site...
http://dakun.art-pulse.net/programs/
the first one is OEcake, as it says :P
you can fiind the official site, with videos and information here
theres no install, and its completly portable. so try it out, and tell me what you think! :D
EDIT: or not... links are down, i have no idea whats up with it right now...