A rendering/game engine I tried making a while ago that simulated non-euclidean worlds, i.e. allowed for "portals". It was never 100% finished but was pretty fun to do. It's interesting looking back on it and thinking of all the things I probably would have done differently.
In its current state, I don't think its's playable. I may come back to it and clean it up to a state where it's at least able to be compiled and used, but last I remember, I was experiencing some bugs with the culling system I was trying to add.