Lately, a lot of leaks from older games with test builds are coming up. After several Duke Nukem leaks, now we have a very early prototype of Left 4 Dead, which is actually made in Counter-Strike: Source as a map called Zombie City. 

This one is actually very very primitive, there are not even zombies or a city at all. This is only a map with some basically built areas. A handful of Terrorist players are against opponent team who are trying to run and stab them with knives. It is not known who leaked the content, but it is a playable mod in Counter-Strike: Source at the moment. 

The goal of this map is fighting through the team of CTs with knifes and reaching to the other end of the level. This was the concept idea behind Left 4 Dead proposed to Valve by Turtle Rock Studios, and getting the project started. All of these led Turtle Rock Studios to be acquired by Valve. It is truly a historical leak, showing us how did one of the most influential multiplayer FPS games started.