Dunno who did these but they are most cool:

http://www.fatfreewater.com/yay/experiment.gif
http://www.fatfreewater.com/yay/trickernation.jpg

View in IE and press CTRL+A (select all) to view.
NB: Those images are not work safe, however the one referenced from this "how-to" article is:

http://www.waxy.org/archive/2003/03/26/hiding_s.shtml

Gareth