Did you hear the one about how a country found a way to get its citizens to work for free by disguising it as a game? Oh wait, that's actually happening. DARPA (the Defense Advanced Research Projects Agency), the arm of the US Department of Defense responsible for new and experimental technologies, has come up with a handful of bug-finding tools to verify the mind-bogglingly huge amount of code the agency generates, and they're disguised as casual video games. One such game, called Xylem: The Code of Plants, is available right now on the iPad.
Xylem tasks you with analyzing new species of flora discovered on a mysterious island. By describing the growth patterns of each plant with a mathematical formula, players are actually building proofs that can identify whether or not a section of code is valid or not. Because of the fact that you're technically volunteering to work for DARPA while playing the game, only users 18 or older can register and play.
It's a system DARPA calls Crowd Sourced Formal Verification, but it's not much different from a number of other web-based tools that act as a game while accomplishing a separate task behind the scenes. Google Image Labeler, for example, was a game where two anonymous players had to find the same single word to describe an image. The words that each user typed when trying to label the image were used to help make image search results more accurate.