In any game that lets you cook grenades, I'll pull the pin and run toward random players screaming "Durka Durka! Muhammad Jihad!"
In any of the Halo games, I love showing my friends the "Blue Face Glitch." This generally only works on players who are unfamiliar with the game. All you do is tell them to stand still so you can show them, and then stick them in the face with a plasma grenade, making their screen turn blue until they explode.
In Super Smash Bros Melee (I haven't played Brawl, and I don't actually own the original...) I'll play as Donkey Kong and use that annoying grab move he has in which he can carry characters on his back and do basically whatever he wants with them. I'll grab some random person and instead of beating the everloving crap out of them, I just walk off the edge of the level, killing both of us. I die a second earlier, so technically if I did this for the whole game I would lose, but it's still funny...
In Forge mode in Halo 3, I'll tell another player to come over so I can test something, and then I simply spawn a tank on top of them.
All of these are really generic, but I still enjoy them to no end.