Something fun to do in Halo 3 whenever I'm bored.
Go into any matchmaking game where you have a team. After the round starts, follow a random person on your team. Shoot at them. Make sure you don't kill them. Once they're close to death, leave them alone till their shield is back up, then shoot again. Eventually, they'll get pissed off and kill you. As soon as the option is available, kick them from the game for TKing you. Rinse and repeat on your other teammates till the end of the round.
This is surprisingly hilarious. I've actually gotten rid of my entire team once. I also got some very angry messages that just added to the hilarity.