Zombie Master is an FPS/RTS mod for HL2. The game is set up as such:
Either you play on the survivors team: You're isolated with your friends against a bunch of zombies, there asen't a lot of weapons around and most are out of ammo, and you have to follow certain objectives to get out of there. You might have to barricade against zombies, or watch out for traps set by...
The Zombie Master: If you play as the ZM, you get an RTS type of view; You control all of the zombies on the map, you choose where to spawn zombies and you can set traps and such. It's epic.
It's probably the most fun HL2 mod I've ever played. Come play?
http://www.zombiemaster.org/