Quasco is a Fienix sub-project that allows you to play Quake mods as stand-alone games even if you don't have Quake installed. All assets in Quasco carry FOSS/GPL or Creative Commons licenses. Quake still receives a lot of content and attention today, but if you're new to playing mods it can seem overly complicated to set up. With Quasco, you can install popular mods like Alkaline and Arcane Dimensions from Synaptic or apt just like anything else.
Alkaline is a game consisting of a central hub with portals to several industrial bases. Each of these is a level with its own unique experience. You can visit levels in whatever order you'd like. I made a video showing some of the level called Tellus Terminus. It's very fun if you enjoy FPS, and the graphics are very impressive!
Alkaline, enabled by Quasco on YouTube

Have fun!
-Casey