Algodoo

Controls are:
WASD keys for blue player
arrow keys for red player

Your goal is to throw your opponent into the deadly acid lake below by pushing him, hitting him, jumping on him, etc.

You can also get a cheap (and somewhat pyrrhean) victory:
Press 0 when your opponent is on the left side of the platform
or 1 when he's on the right side and try to stay on the opposite side for a few seconds.

Have phun!