Zombie Panic!: Source

While we all wait for L4D Steam just launched support for the source mod Zombie Panic. You can get it free from the steam store if you own at least 1 source engine title (Episode 1 available this weekend for $5) - 4 other mods: Synergy, DIPRIP, Age of Chivalry and Insurgency, are also freely available and painless to install.

The idea: Realistically random assortment of weapons, low ammo, Slow but Smart zombies (they can hear your voice chat but you cant hear theirs) - you spend a lot of time having to trade guns and ammo with other survivors ("I have magnum ammo if anyone needs it!") And it just adds to the appeal of the situation you find yourself in. Fighting off zombies with a keyboard: priceless.

The maps are needless to say, confusing as sin. But you do learn them over time - it just takes much longer than your average game, say. 2 play modes: Objective and Survival. Objective requires you to do things in a particular order like activating generators on one side of the map to power up the subway car on the other end that will take you away to safety. In survival, there is a limited number of zombie respawns - when they run out, and the remaining live zombies are killed: you win.

The background music, the dim lighting, and the ever present overmind whispering to you to feast on their flesh makes for hours of entertainment, and it wont be long before you are going back for your second helping of brains.

more info: http://en.wikipedia.org/wiki/zombie_panic_source and plenty of active servers to choose from.