here's a good one....
http://darkartsmedia.com/google.html
change address bar to say 'www.google.com' but do not hit return
call someone impressionable over
tell them you can now 'rub out' the O's in the 'Google' logo on the screen
cover them with 2 fingers and pretend to rub for about 3 or 4 seconds - at the same time, click the left mouse button on a WHITE area of the google page
move fingers away and the O's have gone!!!
do the same again (rub and click) and they with magically reappear!!
one for the kids really, but it's a nice little joke.