Here's documentation on the popular online mahjong client 'Tenhou' (天鳳). What with the entire website and client being in Japanese, English documentation would be pretty useful to novices. Though this doesn't teach mahjong in itself, it should explain Tenhou inside out for the end user. Email for typos, dead links, errors, complaints and such.
If you are interested in how Tenhou works, you probably want my Tenhou Protocol Documentation, which echoes some of the content here from a more technical viewpoint (for anybody interested in implementing a version of Tenhou themselves, the documentation is real helpful). There are also some tools and such to have a look at.
This documentation is split up into various files for different topics. That's basically to make it more straightforward to navigate and write. Initial dates of writing are at the top of pages, while the most recent modification dates are at the bottom.
Lots of stuff in this was translated or explained to me by people on #mahjong/Rizon. Blame them for this too. Direct contributions noted also.
This doesn't cover how to play riichi mahjong, only how to use the online client Tenhou. See Other Resources for a list of tutorials and links relative to learning.
I'm always working on this, while it might not be so apparent. See the ChangeLog for all edits done, and TODO for stuff that I plan on doing. Please also note that this documentation and resources are all under an Attribution-Noncommercial-Share Alike license.
Lobbies and Games
User IDs and Ranks (Rate)
Purchasing an ID
Frequently Asked Questions
Network Protocol Documents and Resources
Glossary (including yaku, game modes, ranks and rounds, among other things)
ServicesGame records/statistics database
Tenhou Protocol Documentation and resources
Japanese mahjong wiki
Hand Score Quiz and Yaku Quiz Java applets by UmaiKeiki
Mahjong message board for tournament registrations, discussion and whatnot
Tournament results in plaintext form
Game records, updated every 30 minutes
Houou game logs, updated twice daily
Wait Calculator notes (very rough notes)
LinksBlog I run on technical mahjong related things (
Tenhou main page
L7447 Tenhou English lobby
Osamu's mahjong blog
"So you want to play Mahjong" Riichi Tutorial - friendly riichi guide, .doc form, teaches Ari Riichi
Modern Japanese Rules - terminology, expanations and hands/scoring on riichi
Toudai-shiki Mahjong Scoring - very comprehensive Mahjong scoring page, has lots of terminology too
Communityfirstname.lastname@example.org - Mahjong IRC channel
#email@example.com - osamuko.com IRC channel (you are more likely to find mahjong talk going on here)
Mahjong message board for tournament coordination, discussion and whatnot