I agree with David Edery that there’s free labor in video games, but he draws an awful analogy in his intro:
Consider the following: in 2003 alone, nine billion person-hours were spent playing the video game version of Solitaire — enough to create 500 Panama Canals.
The famous ESP game is much more like Solitaire:
The game works like this: two anonymous players are matched online without any means of communicating. Both players are shown an image (for example, a flowering plant) while a clock counts down. The players must then type words that describe the image, such as “plant.” When both players have typed at least one word in common, they both score points. More importantly, the players have also unintentionally taught the computer that the picture contains a plant!More than 20 million labels have been harvested by the ESP Game in just a few years — the equivalent of several million dollars of free labor. Professor von Ahn estimates that just 5,000 people playing for a month could label every image on the web. Notably, Google has adopted the ESP Game and renamed it the Google Image Labeler, which anyone can play here.