Added to the lair: 2/17/18
Developer: Rewolf Software
Wikipedia: Gunman Chronicles
It's high noon in deep space
Gunman Chronicles began, much like Team Fortress, Counter Strike, and Day of Defeat, as a Half-Life mod that garnered enough attention to warrant a standalone retail release. Unlike those titles though, Gunman Chronicles seems to have mostly faded into obscurity over the years, which is a damn shame because it's a damn good game.
The story is fairly ridiculous, but that doesn't stop it from being an enjoyable ride - it has a wide variety of levels and enemies, and the weapons are unique in that they have several different firing modes and a number of ways to customize them to your needs. The writing, though often silly, seems to be fairly tongue-in-cheek and is pretty humorous.
You fight aliens, robots, space cowboys, and dinosaurs; what's not to love?