Rich Gain of Microcubology has made the worlds smallest puzzle(as far as I know). You can check it out here in his Shop. The puzzle is $9.95 and is 3.5 by 5.5 by 0.9cm and looks very challenging. Rich has made a video where you can see him playing with his puzzle below. You can solve it in two ways by building a cube or by building an "inside out cube" that is hollow on the inside. Pretty nifty, ya think?