http://www.zombiemaster.org/about.php
It's still in beta, but I have the feeling Valve might snap this one up before it becomes finished.
It's really, really good. Potentially, it's massive. Even now it's hard to get into servers - simply because they're all constantly packed full. It's got a few bugs and is sometimes horribly laggy.
Basically, there's an objective based map. Probably the first time you play, you'll play the docks where all the players (except one) have to find the parts for and re-assemble a truck to escape. Like CS, when you're dead, you're dead for the round, and there is no health lying around.
One player is chosen at random at the start of each game to be the "Zombie Master" and that person plays the whole round like a RTS game, such as Command and Conquer.
Being Zombie Master is confusing at first, but it's a lot of fun when you get it worked out.
Anyway, the link above will tell you all you need to know. I recommend that you have a go!
I was lucky enough to grab a fleeting space on one of the official servers where the dev team were playing. It was interesting - they were much better than me tho
Mind you L4D isn't really the same so...
