I found a Lego ™ rendition of an Escher painting a while back; my mother, being the geek she is, managed to hunt down an archive of the site, which has several other Lego ™ Escher models. Some have photomanipulation, for obvious reasons, but they’re all pretty nifty. They’re near the bottom of the Lego ™ page, so scroll down.

Edit 31 May 2006: The cached version of the site seems to have vanished, but you can see images of one of the models here, here, and here.