PlanetPhysics is a virtual community which aims to help make physics knowledge more accessible. PlanetPhysics's content is created collaboratively: the main feature is the physics encyclopedia with entries written and reviewed by members. The entries are contributed under the terms of the GNU Free Documentation License (FDL) in order to preserve the rights of both the authors and readers in a sensible way.
PlanetPhysics entries are written in LaTeX, the lingua franca of the worldwide mathematics community. All of the entries are automatically cross-referenced with each other, and the entire corpus is kept updated in real-time.
In addition to the physics encyclopedia, there are books, expositions, papers, and forums. You also might want to check out encyclopedia requests if you'd like to see something we don't have.
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PlanetPhysics runs Noosphere.
More info: Wikipedia