Have a look at http://www.openrt.de it's a OpenGL like API ( in fact its just RT instead of GL ) that uses a distributed raytracer in conjunction with a cluster. Very impressive technology and it's a full raytracer, even supporting dynamic objects. And the scene size is independent from memory, not like the gpu approach wich stores all information in the graphics memory. And in fact they're developing a chip to support their algorithms .) It will be very interesting to watch those approaches in the future.
There are also some nice screenies there!
There are also some nice screenies there!