Kaya Go Server
The Kaya Go Server (formerly located at http://kaya.gs) was server developed by Gabriel Benmergui 6d and Patricio Reboratti. The Kaya server aimed to be more than just a place to play go. The developers envisioned an online environment "where everything related to Go is present". Online play, broadcasting of professional games, live video feeds, teaching, lectures, tsumego, social chat, and more!
In February 2012 an Alpha version (monetary donation required for access) of the Kaya server went live, a Beta version followed. In 2013 it collapsed, a.o. due to Gabriel moving abroad and starting to work for a company: http://kayags.blogspot.be/2013/10/about-kayags.html
Kaya was browser based, thus not dependent on a client that may not be available for all operating systems.