There are plenty of multiplayer games that have their own 'game' that were made by people of the comunity that play these games, heres just a few of some of the best
Call of Duty: Modern Warfare 2
Micheal Myers - On a Sreach & Destroy game there person who is "it" or Michael will go to the Spetznaz, Ultranationalists, or Opforn and the other people will be TF141 or Rangers. Micheal can only use knifes and the TF141 can only fight back after the one of them is the last person alive. TF141 or Rangers can alsostun Micheal. Also no planting bombs.
Knife Tournament - 3 lives and knifes only. Who ever can score the highest wins. Throwing knifes are allowed.
No-scope Tournament - Simple kind of like the first, but you have unlimited lives and you have to no-scope
Grand Theft Auto IV
Prison Break - Before you start the game while in the lobby put the location at the Prison. One team is the 'Cops' and the other team are the 'Inmates' the cops have to go to the nearby police station and pick up a cop car and wait for the 'Inmates to leave the prison. If the 'inmates' make it to the airport they win
Zombies - In Team Deathmatch one team uses the 'Zombie' skin and the other are the survivors. The host needs to put the cars to parked and pedestrians to off and the weather to foggy. Nobody can use cars and you can only run if the 'Survivior' team lives for the full time limit then they are the winners. If the Zombie team manages totake them ALL out then they win
Carmageddon - In Free Roam take a helicopter to Happieness Island and if a car spawns in the very back edge take it. If it does not then comeback and try again later. You have 2 minutes to see how many people you can kill all around the island. For crazy people only :P
Failed Superman (TBoGT only) - Use the parachute to jump off of a sky dive building and pull your parachute at the very last possible second and live.
Left 4 Dead
Bommerball - Make a friends only campaign game mode and make sure one of your friends knows how to spawn items. Once you have a blocked off confined area type the console command z_spawn bommer to make a bommer appear, keep knocking the bommer back and forth until it explodes, the person who causes it to explode must be shot by everyone on his team by a shotgun.
Pacafist Mode - Play through a entire campaign game on ANY difficulty without killing anything. Very very hard game mode :P