Sumotori is a physics sumo wrestling game. it's not online, but you can play with a friend on the same keyboard.
You have to buy it however. but it's only 5 measly bucks. Very affordable. Sumotori is moddable also, you can make maps and such.
http://www.gravitysensation.com/sumotori/GameplayOh, and if you play the full version without a key, then you only get 7 seconds worth of playing time.
