Here is a small, freeware single / multiplayer FPS called "Innawoods" that was initially made in 48 hours by Armitage Games. They've since made some revisions to the game. The premise is that you're in the woods being hunted and must fend for yourself while fending off your adversaries. It features a minimalist art style and appears to have Quake 1-level engine capabilities.
http://gamejolt.com/games/other/innawoods/11484/