Just StumbleUpon'd this one
http://www.scientific...BallShapiroIC2009.swf which I haven't seen before.
A "spinning" ball drops vertically but if you look to one side it seems to travel to the left - there's a blue dot on the right as a guide but it doesn't matter if you look to the left side instead. And if you change the settings then... (try it yourself).
There's a lot of age-old optical illusions around but what others are there that a computer can show for which there is no equivalent printed version?