Zombie Master (or ZM for short) is a multiplayer Half-Life 2 modification centered around a zombie-apocalypse, which throws all but one player in the shoes of human survivors. Pitched against a relentless tide of rotting flesh, they must scavenge for weaponry and equipment just to stay alive. But that one player, chosen randomly at the beginning of the round, is not bound by mortal restraints.
To speak his name is to invite the wrath of hell - but most call him the Zombie Master. He is nigh omnipotent. His invisible hand controls the legions of undead like puppets on strings; slowly closing the net on the humans while they run to and fro like rats in a maze.
Zombie Master was started in late July 2005. The first public release was on the 15th of April 2007, and the mod was updated regularly until the last release by the original team on the 4th of January 2009.
The project's code was open-sourced on the 13th of June 2009. At the time of writing there are several teams/people from the community working on patches and updates to the mod (the progress of which can be followed at the ZM forums).