Zombie Master predates Left4Dead by somewhere around 2-3 years. The concept is roughly the same, with a number of key changes:
- There can obviously be a lot more people on a server than just four survivors.
- There are no "boss" zombies; there are, however, five Zombie "types": Shambler, Drifter, Banshee, Immolator and Hulk.
- Whereas Left4Dead has the "AI Director" that intelligently monitors players and spawns zombies according to their stress level, Zombie Master has a human player who views the map from an overhead perspective (like an RTS) who spawns zombies and controls special map events.
With the right group of people and a good map, it can be an incredibly good time.