The freaky Half-Life Mod Zombie Panic has just released their V0.91a Open Beta client as a special Halloween treat! :D
The basic idea of the mod is straight out of many different horror films: Zombies are coming for the last remaining humans, and the humans must fight them off as best they can.
Everyone joins a server, say 10 people. All of the players but one join the Survivors team, which is the humans. The remaining player joins the Zombie team. His goal is to kill all of the humans. The humans' goal, of course, is to stay alive as long as possible. The catch is that everytime a zombie is killed, he simply respawns and continues to play. If a human is killed, he respawns as a Zombie on the Zombie team. Soon, more and more humans have joined the ranks of the zombies, until there is only one human left, facing all of the zombies alone. When the last human dies, the round is over.
Happy Halloween from the HL2Files.com team! :D