Check out the amazing 3-D artwork of pavement artist Julian Beever. His chalk pictures are drawn so that they appear 3-dimensional when viewed from the correct angle. In this photo (copyright and used with Julian's permission) the man, I presume the artist, is standing on one of his pavement pictures - you can still see the paving slabs.
Julian has been around for a while, but I have only recently come across his work, so I'm guessing others may not have seen these pictures either. Check out his website, or go direct to some of his best works including: