Why, a Raytracer, of course. If you aren't familiar with exactly what raytracing is, then I strongly recommend you take a read-through on the aforementioned link.
Writing a raytracer has always been a huge ambition of mine -- it's just so cool to think that such a simple algorithm can produce incredibly realistic 3D images with such ease. Not to mention the utter awesomeness of being able to define this world using shapes composed of raw mathematical equations...
Read the rest over at the new Dev-Journal location.