I've played a few different version of Zombie mods in Cod4. One was one player started off as a zombie, and had to "infect" all the survivors. If the players were good, it would take forever to kill them as they always were loaded down with extra ammo (bandoliers), and other perks to help increase their survivability. Was a lot of fun, but boring if there wasn't enough players (say at least a group of 8). Really bad on the easy to camp areas, where they only had to knife you before you could attack.
The other mod was one with Zombie classes. Provided a different experience, especially when the Survivors had to deal with range attacks from zombies, as well as zombie-dogs. Personally, I liked the first mod more as it forced zombie players to be creative in their attacks. And it really shined depending on how great the map was.
I'm pretty sure there still are zombie mod servers out there. You download it when you connect to them. Just sort the gametype by name, and scroll to z_mod or something similar.