Starting this Monday, Valve will be making Alien Swarm Unreal Tournament 2004 mod available for free via Steam as a full, Source-powered four player co-op. The team that developed the title actually went on to work on many popular Valve titles such as the highly anticipated Portal 2 and the Left 4 Dead franchise.

In addition to releasing the title for free on Monday, Valve will also be making the entire code base for the game available to devs, as well as all modding tools. This includes a tile-based map editor. Unfortunately, it doesn’t look like their will be a simultaneous release for Mac users.

Valve announced on Steam:

“we never forgot about Alien Swarm and the team has spent a lot of time bringing the game to Source in between their contributions to the other Valve projects.”

You can download Alien Swarm for free via Steam starting Monday.

[via Steam]