Escher was a Dutch graphic artist known for mathematically-inspired woodcuts, lithographs, and mezzotints. These were often impossible constructions, explorations of infinity, architecture, and tessellations.
The great thing about them is that they're impossible constructions. Or at least, they were until Andrew Lipson and Daniel Shiu built them out of Lego. I like that in the top one, the lamp is resting on a Linux book. Rather heartening.