What are you're favorite FOSS games to play and why?

I love Endless Sky, it's honestly one of my favorite games! I have a pack of cool SciFi art that I found on Nexus and I've been slowly replacing the stock photos for my personal game. I'm also a sucker for TeeWorlds, it is delightfully infuriating and hillarious to play. I know that's baby's first FOSS games but I'm new to open...