The ASR - Advanced Study Room is a non-profit global GO community which runs by volunteers. It has its own league system, the so called ASR league with five league divisions (alpha, beta, gamma, delta and placement league).
Sensei's Library has an info page about the ASR League as well, it was formerly used to organize the monthly league, but that is a long time ago. Actually the whole league system runs independently on an own server (Debian Linux 8, Apache + MySQL) as documented on the ASR website.
Some older games from the ASR - Advanced Study Room can be found here