The still active Quake-engine scene is probably a bit under-represented here, but I wanted to highlight some cool little game jam titles a modder called shpuld it making with the FTEQW engine on regular basis:
There are some quite playable games among them and source-code is always included (but not always on the github page). Art assets are not all libre licensed, but mostly quick game jam placeholders anyways.
I think some of these have interesting ideas and could probably be picked up by others to further expand. FTEQW is also a quite modern engine (at least by Quake standards), so it can do a lot of cool stuff.