(Thanks to Anders Claesson for this pointer.)
Luis von Ahn is the winner of one of the MacArthur genius awards for 2006. I just tried his ESP game ( http://www.espgame.org/) for the first time. It is actually rather smart, in a way, and apparently some people play it a lot (even over 10 hours a day, according to von Ahn). In so doing, they help label images on the web, using all those human brain cycles devoted to game playing for a good cause.