I actually finished something for Ludum Dare, it's entered in the game jam section (72 hours instead of 48, and relaxed rules). It's a first-person puzzle game, you can slow down light to 2 m/s, the puzzles are very simple right now since I didn't have much time to work on them. The engine is a 4D raycaster, the world is stored in a 4 dimensional voxel grid, as the ray moves though the spatial grid it also moves backward in the time dimension. There's a lot of cool stuff that could be done with this with some more time.
Try it out here: relativity.zip