Tenhou.net
Tenhou (天鳳, tenhou.net) is the one of the most popular online mahjong service in Japan. It is also the platform of choice for the English-speaking community.
User's guide
Comprehensive English documentation can be found at http://arcturus.su/tenhou/.
History
2006/02/20 Hanjyukusou (半熟荘) beta released 2006/08/01 Came out of beta 2006/10/24 10,000 IDs registered 2007/01/20 Changed name to Tenhou 2007/12/25 100,000 IDs registered 2008/02/01 Windows Client released 2009/02/20 Houou table opened 2010/08/22 1,000,000 IDs registered 2012/06/09 10,000 concurrent users 2013/07/10 2,500,000 IDs registered