With a little bit of experimenting, someone just found out that if you stack minecarts in Minecraft, you can create an endlessly sliding object that never actually stops. Build an ice rink around it, invite some friends and you got yourself your own hockey game.
Sethbling built a rink for two players, but this would also probably work easily with multiple teams and in a big arena for enhanced fun. To even further tinker with the idea, the carts could also be used as a steel ball for some weird Minecraft pinball machine. That would be a cool build.
Minecraft Air Hockey [YouTube]