Escher
Site contains examples of Escher's work
http://www.dartmouth.edu/~matc/math5.pattern/lesson7art.html

Math Forum - Tessellations
http://forum.swarthmore.edu/sum95/suzanne/tess.intro.html#tools

Tessellations
http://ois.unomaha.edu/cdeval/neesman/tessel.html

Tour: M.C. Escher -- Life and Work
You will find approximately 80 examples at this National Gallery site
http://www.nga.gov/collection/gallery/ggescher/ggescher-main1.html