The classic multiplayer zombies vs survivors game. As the human players are killed they join the ranks of the undead - a conceit that works as well in a game as in the movies. The round is over when the last human player dies.
Your standard slow-moving rotters, but they are relentless and respawn continuously. Once human players are at the controls the fate of the in-game humans is pretty much inevitable. Nothing worse than a smart zombie.
Human survivor players are able to get hold of weapons to hold off the zombies, but the more ammo they carry, the slower they move. Moving slowly near zombies is never a good idea. Balance is everything.
Zombie Panic has already been a notable success as a Half-Life modification, and given its eager fanbase, its Half-Life 2 Source-based variant is bound to be just as popular, even with Left 4 Dead incoming.