FWIS3

"A Slower Speed of Light" is a relativistic effects simulator with atmosphere, courtesy of MIT.

http://gamelab.mit.edu/games/a-slower-speed-of-light/

The hardware requirements aren't as bad at lower resolutions. My AMD A2 workstation can handle 800x600 without issues (though it chokes on the default resolution).

Edit: actually 1280x1024 (the maximum for the workstation's monitor) also works okay. I think the hardware requirements are vastly overstated. (Especially as the workstation has an onboard GPU, not dedicated.)

Where are the Zombies?

No zombies.

There are other spirits who glide around in robes and white masks, though. I think more of them show up as the speed of light decreases. Unfortunately they don't talk or otherwise interact with you.

(And actually I'm not sure if the time dilation effects are correct for them? They don't seem to slow down when you move, but it's hard to tell with all the blueshifting color weirdness.)