Zombie Panic! is essentially a zombies vs. human survivors mod, with the zombies trying to wipe out the humans, and vice versa. The catch that makes it fun and interesting is that all players except one join the human team at the start of the round, and only one person starts off as a zombie. The zombies respawn when killed, but when a human is killed he respawns as a zombie, and joins in hunting down his former friends.
If you would like to view other available addons, we suggest you browse the addon list.