This is probably one of my favorite games that is completely open source. It's a collection of fan-made first-person stealth missions and campaigns that take heavy inspiration from the classic Thief games. The name comes from its origins as a mod for Doom 3, but it's been released as a standalone game for quite some time now.

If you enjoy the old Thief games, give this a shot!

I figured I would kick the community off with the game I am currently playing. SS14 is a lot of fun! Many different game types!