-- Math for the people, by the people.

From the website (hardlinks are by me):

The idea for the project was hatched by unlord, sometime around fall 2000, in response to the unfortunate removal of the great resource, "Eric Weisstein's World of Math" (or "MathWorld") from the internet. This action created a void for a useful, comprehensive math encyclopedia freely available online, which we wanted to fill as quickly as possible.


We also wanted to make certain that the users of PlanetMath would never have to worry about the removal of the content they had contributed in favour of commercial sale. So it is the express goal of PlanetMath to be an internet resource, first and foremost. We hope to achieve this with the aid of the GNU FDL, or Free Documentation License.

The structure of the site resembles Everything2 in that any user can post an "object" into the database. It does seem to have many differences, though (corrections and comments come to mind). One of the most important features is that submissions are written in LaTeX rather than HTML -- you can even upload diagrams to display in your objects. The end result is a very easy way to write up a document that uses any sort of mathematical notation.