http://www.zombiepanic.org/site/index.php
This HL1 mad has just been released, its a pretty good idea. Most of the players start on the non-zombie team, you can pick up various guns but ammunition is very limited.
At the start of the round, one person is a zombie. As a zombie you can die as many times as you want and will always respawn almost immediatly, if you kill someone who is alive they will respawn as a zombie and remain so for the rest of the round. This means that as more people die the number of zombies increases and the number of teammates decreases. This creates some very cool gameplay as, to succeed the zombies need to stick together, which they usually do, so any players still alive need to fight off waves of zombies.
I did a shitty job of explaining it but trust me its a damn good idea.
Definatly worth trying as its only about 30mb and there are a few servers.
Also i was thinking this would make a brilliant doom3 mod. Someone needs to make it pronto.