M C Escher's work has always facinated me. i stumbled across a very cool java applet which lets you try and make your own escher type drawing. I've put a link on the picture, the header and below in case you're interested in trying it out!
the link below will take you the officially maintained escher site..
This picture is a b&w one but you should see the combos used by escher - really outstanding
Pa you will enjoy this site.