Produced by MIT Game Lab, A Slower Speed of Light is an open-source “game prototype” that takes players on a (very) short tour of the relativistic nature of light.
It’s more a demonstration than a game, though. Players are tasked with collecting floating spheres, and for each sphere they collect, the speed of light slows down.
As it approaches your character’s walking speed, you can witness the effects of special relativity: the doppler effect, time dilation, and the ever-disorienting Lorentz transformation (or the “warping of space at near-light speeds”).
“A Slower Speed of Light combines accessible gameplay and a fantasy setting with theoretical and computational physics research to deliver an engaging and pedagogically rich experience.” – A Slower Speed of Light homepage
Definitely worth a look. You can download it here, and there’s no need to install anything. Just extract the .Zip file and run the app (there have been some reports of crashes, etc., but it seemed to run okay for me).
Oh, and the developers are also making the game engine open-source (called OpenRelativity), hopefully allowing other game designers to use the engine in their own games down the line. Should be a trip.