http://www.eyetricks.com/illusions.htm
http://www.optillusions.com/
http://www.liquidgeneration.com/sabotage/optical_sabotage.asp
http://www.ritsumei.ac.jp/~akitaoka/index-e.html
http://members.aol.com/Ryanbut/illusion1.html
http://www.candleshoe.org/opticalillusion/index.shtml