[iZN]

There aren't much horror games, but I don't know if zombies are actually horror-type, I always loved Left 4 Dead 2. Then if you're looking for custom modes and stuff, check out Garry's Mod, it's basically like SA-MP with alot of different custom modes where you can play with your friends and other players (just like other Valve games), there are many horror modes out there which are quite enjoyable with friends over a microphone.