Anything built in an ad-hoc, bottom-up manner by a collection of individuals who are more-or-less peers. (cf. p2pfoundation.com)
Research has shown that a crowd of average people creates a mean that is better than the average expert. (citation to look up: guessing the weight of cows at a fair).
Alternatively: Data, services, content or feedback obtained from the general public, or the use thereof, typically via the Internet. E.g., "We crowdsource articles for our Website."