Half-Life: Threewave

Threewave began life as a Capture the Flag mod for the original Quake that went on to enjoy huge popularity among the PC first-person shooter community.

Background
The original mod for ID's software Quake came out in 1996, and was one of the first capture the flag FPS shooters of it's kind. It's creator Dave "Zoid" Kirsch was hired by ID to make the Capture the Flag mode for Quake 2, before later joining Valve where he still works today. The mod was abandoned by Valve and ID at some point for seemingly unknown reasons, It was first found by a Youtube news channel, Valve News Network who later spent time on revamping to mod to a playable state.