well I didn't know about this until yesterday, although it's been released in last year
http://decay.half-lifecreations.com/
it's basically a port of the game done by Gearbox on Playstation 2. The game is supposed to be played by 2 players in a cooperative mode, however the PC port can be played with 1 player
playing it with 1 player is a bit annoying though IMHO - as the other character can't follow me automatically and also doesn't always shoot unless I switch to take control, especially when following someone it's too easy to get lost (and it's a gameover if the other character dies btw)
Maybe implementing something like Final Fantasy 12 (i.e., a party consists of 3 members is controlled by one player however player can turn on/off the AI for any party member, and player can switch to take control of any member in any time) can be much better
