MMO where up to 6 players can go around and raise hell together, or against each other in PvP.
You can enter buildings, barricade them and make them bases, or strip a building of all it's valuables and leave. Or, just go around killing zombies. At a certain time every day, the outpost(main base) will come under attack, and you can defend from the rooftops, or the frontlines. Bunch 'o different classes and such. Good time waster.