Valve actually used cdak (http://www.pouet.net/prod.php?which=55758) to demo their prototype headset.

It was a specially optimized version to be able to run at full framerate on the hardware of that time. I have looked for it but it doesn't seem to be available anywhere.

There is a Shadertoy version but without the optimization and last time I tried I actually had to tweak it a little to make it compile.