Most of the render engines out there use 'biased' rendering. Basically this means that the render engine will try to simulate a realistic effect and thereby saving a lot of time. Actually it is kind of cheating.

"LuxRender simulates the flow of light according to physical equations, thus producing realistic images of photographic quality." The effect can be stunning indeed but this increases render times drastically.

LuxRender is free of charge and runs on MacOSX, Windows and Linux. Currently it works with Blender, Softimage, Maya and Cinema 4D.