I just translated from French to English an old page on my website about a real-time GPU raytracer I developed for fun 4 years ago, during my Master Thesis. It is old school GPGPU in OpenGL and Cg that can run on an NV40 (GeForce 6800). No need for CUDA or a GF110 to do GPU raytracing ! ;-)
The application also features a slow and unoptimized CPU raytracer.See there: http://www.icare3d.org/myprojects/opengl_projects/raytracer_gpu_full_1.0.html
PS: It is funny to see what was possible at this time, but it was developed quickly and the shader code itself is not a reference !