Raycasting takes a light source and simulates thousands of rays coming from it bouncing off the walls of a room hundreds of times. In the process, you produce very realistic lighting with cast shadows and everything that you would want. This process is computationally expensive so the technique has been used to create prerendered lightmaps.
In this program, you can actually walk around the room, and take a close look at the soft shadows that are produced by the 1 million rays cast out across the room. Yeah, that's right, 1 million. I needed a few more than I originally expected. The reason I can get away with this is that the 1 million rays have been calculated on my computer and lightmaps have been rendered. So all you need are the lightmaps, which is very simple to implement. The result is a very fast demo of radiosity rendered lighting.
P200, 32MB, with a decent video card that supports OpenGL.