A term used to describe situations in which there is a "commons" (communal resource) which is made more valuable by usage. Compare to TragedyOfTheCommons, in which usage "uses up" the commons.
Coined by Dan Bricklin back in 2001 to describe the success of Napster. The idea was quickly used to describe the way the FreeSoftware movement worked; wikis were eventually also mentioned, and so was SocialBookmarking.
There's a difference between cases where the resource gains value by the sheer fact that there are many users (such as napster, or message boards), and cases where more people paying attention means possibly more volunteers (as is the case for free software projects). Wikis tend to vary between those extremes.