There's a list on Wikipedia of zombie-related games. Unfortunately, I seem to have played most of the free ones.
Nobody ever seems to get the genre right... It's always some linear FPS (or TPS in the case of R:E), or it's a mod for Half-Life 2 that really should've had a bit more time spent on the development front.
Do you know, I have never once seen a zombie RTS? I've thought about the idea several times, where you have three different factions fighting over a map that's infested with computer-controlled zombies.
You'd have punks, the cult, and the army remnants. Punks would be plentiful and good at looting, but not individually effective in a fight. The army wouldn't be too keen on looting (at least not for weapons) and would be very good fighters, but in very few numbers. The cult would use zombie-centric traps in order to deal with the other survivors. I'm not saying that the zombies would ignore the cult, it's just that the cult would try to use the zombies instead of killing them off.
You could have day/night cycles, the ability to garrison buildings (although no RTS seems to have mastered the intricacies of such), and other fun stuff. So, yeah... That's an old idea I used to think about.
Survival Crisis Z is the closest I've seen to the kind of zombie RPG I want, but it is still a fair ways off. I don't think that RPG exists quite yet.
I've also thought about an idea for another HL2 mod. Singleplayer mostly, but with multiplayer co-op missions.
The game already has several animations, atmospheric designs and voice reactions for a zombie menace. The tricky parts would be making new zombies with their own animations and skins, and adding some sort of inventory. I imagine a singleplayer campaign that ends in a building-crowded city district that's swarming with zombies, and having a strike being called in to some orbital thingy (I suppose you'd have to fit aliens into the game somewhere along the line, or at least a top-top secret experimental military test station) whereupon a series of scripted combine mortar strikes will rain down upon the area, disintegrating the zombies and leveling nearby buildings.
Um, yeah... Don't mind me...