"PlanetMath is a virtual community which aims to help make mathematical knowledge more accessible. PlanetMath's content is created collaboratively: the main feature is the mathematics encyclopedia with entries written and reviewed by members."
From the abstract: "is research develops around a technological intervention intended to transform
a peer produced reference resource into a peer produced learning environment. An
early Web 2.0 community for mathematics, PlanetMath.org, will henceforth become a
mathematical practicum, and a laboratory for learning science."