Like alotta geeks M.C. Escher is one of my favorite artists. His paintings play on perspective and twist realities.
This evening after reading the Wikipedia entry on him, I found an excellent gallery of his drawings. So go there and boggle at his amazing abilities.
I was looking for this one in particular (I would put it directly on this page but dont want to infringe copyrights...or something). Somewhere I have a copy of this print, one of the few things I used to display in my room.