M. C. Escher worked by hand but some of the transformations he used were entirely mathematical, and so the same can be applied by computer to digital images.
Conformal Mappings - a photoset on Flickr
I've always found Escher's work fascinating and I could look at it for hours, but the nagging question that always lurks in the back of my head is how much of it is simply clever and how much of it is art? or is that cleverness a form of art? For instance, I can't see much that would be interesting about "Print Gallery" without the visual twist.
The first of those two Flickr photos above I would call "aesthetically pleasing", but the second I'd hesitate to do the same. Yet I do find it fascinating, but it doesn't seem the same kind of fascination I'd feel for a work of art that was beautiful without being clever.