Escher Web Sketch is a nice applet that allows you to draw repeating patterns à la M.C. Escher through the use of different types of crystallographic symmetry. The page also has a couple of links to sites about M.C Escher, one of my favorite artists. I’m continually amazed at his work, especially at how he managed to master such complicated symmetries and perspectives and utilize them in creating such mesmerizing art.
