I was trying to think for ages who it was who did those pictures that mess with perspective. I was only when I saw stairs moving around strangely and the mention of an Escher Vault on a Warehouse 13 advert on Syfy that I remembered. His work is amazing, so check it out if you happen to have never heard of him before.
[Wednesday, 27th October 2010]
Edit: This is a tracing of Escher's "Relativity", which I did in the past week to get to know it a bit better.