Yafray is a free, cross-platform, open source raytracer, meaning you can download it, modify it, and use it to make computer graphics. It uses XML for its file format, which is probably a good thing. At the time of this writing (December 2002), its development is fairly active. This is a bit unusal, as many promising raytracing projects seem to be abandoned halfway through. Like all text-based ray tracers, it's a bit tricky to use without a graphical front-end; luckily, there's an exporter from Blender (a cool 3D package that is also free, cross-platform, and open source). This is fortunate for Blender users, since Yafray does fancy raytracing and photon mapping that Blender's fast but less sophisticated renderer can only dream about. So far, it's mainly been used for making pretty pictures and other eye candy. Oo, pretty shiny raytraced monkey heads!*
Can't wait to find out more? The project's current website is http://yafray.org/ .
*Yes, Blender users often use monkey heads for their rendering experiments, because from version 2.25 and onward, "ADD > Mesh > Monkey" entered its primitive shapes menu.