From Valve Developer Community
BrainBread is a co-op modification for Half-Life.
An arsenal of over 20 weapons makes sure you stand a chance of surviving against the zombie hordes - even if it's but a small one. Are you afraid of being killed by a zombie? One thing is worse: becoming one of them! Avoid getting bitten by those bastards. The Army is your only ally. AI based tanks, helicopters and soldiers will bring great AI against AI fights onto the streets!