Now that Turtle Rock Studios is no longer working on Evolve, it has some time on its hands... and it's giving veteran gamers a treat. If you have a PC copy of the original Left 4 Dead (one of Turtle Rock's best-known games), you can now install Dam It, an unfinished but playable free campaign that was meant to connect two of the shipping game's stories (Dead Air and Blood Harvest). You start at an airfield and have to make your way through an apple orchard, a ravine, a covered bridge and a campground before a climactic fight at the campaign's namesake hydroelectric dam. Unlike most other L4D tales, you're actually encouraged to split up -- you just need to stay close enough to offer support in a pinch.