Sensei's Library is a type of collaborative online project, known as a
Wiki, similar to
Everything 2 in some ways. Anyone can create a page, and
all the content of a page is editable by anyone, even if they didn't write it themselves.
The primary difference between Sensei's Library and E2 is that, while Everything is about, well... everything, Sensei's Library is devoted to the game of Go (Wei Qi/Badouk). It covers all aspects of the game, from the rules, to general strategy, to Go-related humor, to biographies of professional Go players. The editing system includes a convenient way of displaying a whole or partial goban (Go board) to illustrate arrangements of stones or sequences of moves.
The vast majority of my Go knowledge (which I'm now in the process of noding) was learned from that site, since I'm living in South Korea and can't get my hands on any English-language Go books.
Sensei's Library can be found at http://senseis.xmp.net.