So I stumbled across this sourcemod called "Shotgun Sunrise". It's a co-operative western shooter with AI-controlled zombies. The game takes place in a single huge map of the town "Shotgun", and you drive around with your friends in a truck capping zombies and completing objectives.
Anyway, I tried it with Solar earlier today and I found that while some aspects lack polish it's a really fun co-op experience.
The game actually needs a minimum of two players to complete, so make sure you have a friend download it with you if you are going to try it.
Edit:
Download link:
http://www.moddb.com/mods/shotgun-sunrise/downloadshao2install:
1. Extract the downloaded rar onto your desktop
2. Drag the Shotgun Sunrise folder into sourcemod folder (default C:\Program Files\Steam\steamapps\sourcemod)
3. Restart Steam.
4. Profit.
might be able to just extract SS into sourcemod folder and just restart steam from there
