Here is a rough timeline of online mathematical experiments:

1995: Eric Weisstein puts up his Treasure Trove of Mathematics, which would later become Wolfram Mathworld. This is largely his own work.

19992000: The Treasure Trove of Mathematics migrates to Wolfram, becoming Wolfram Mathworld. See our backgrounder on Mathworld.

20002001: After the Mathworld website goes down because of a lawsuit by CRC Press, Aaron Krowne starts work on a collaborative mathematics encyclopedia to fill the Mathworldtype niche. This is called Planetmath. Work on Planetmath continues even after Mathworld is back. See our backgrounder on Planetmath.

2001: Wikipedia, the free online encyclopedia, begins action. In the beginning, it has very little mathematical content.

20082009: The members of the ncategory cafe (a group blog on higher category theory) start a wiki called ncatlab (or nlab) devoted to the themes of higher category theory in the larger context of mathematics, physics, and related subjects. The entries in nlab are meant more as public workbooks than as encyclopedic entries. See our backgrounder on nlab.

20082009: Tricki, the Tricks Wiki, gets started. See our backgrounder on the Tricki.

2009: Polymath projects, which are massively collaborative attempts to solve mathematics problems, get started. See our polymath backgrounder.

2009: Math Overflow, which is a place for asking and answering questions on graduatelevel or higher mathematics, gets started. See our Math Overflow backgrounder.
A lot of your links are broken. I ended up finding the “backgrounde”s on your sidebar, though.
Comment by bugfinder — December 28, 2013 @ 8:24 pm