Information about mahjong is so easily available now on the Internet. Anyone who wants to learn mahjong can find a lot of materials online, although not every major style or variant is well represented.
For a good general website on mahjong, there is Sloperama's Mah-Jong Zone, run by Tom Sloper, a game designer and mahjong teacher/writer. In fact, Tom Sloper has just published a book on mahjong, The Red Dragon and the West Wind, which is on both Chinese Official/Mahjong Competition Rules (the Red Dragon) and American Mah-jongg (the West Wind). There are FAQs on every aspect of mahjong; a column with tips on strategy and practice questions on scoring; and a bulletin board for readers with questions. It is a very good resource, with lots of information and links.
Mahjong News is a European-centric website with frequent updates on mahjong tournaments and developments in Europe. For example, the European ranking list of Chinese Official mahjong players can be found here.
The much talked-about Mahjong Competition Rules (which is actually the most recent version of the Chinese Official ruleset) can be downloaded in PDF form from the World Mahjong Organisation's website. Recent tournaments such as OEMC 2007 follow this version where some early problems and inconsistencies were corrected.
Alan Kwan's webpage has some articles on mahjong playing and mathematical analyses of tile combinations used in (various variants of) mahjong. Alan Kwan is the designer of the Zung Jung system which was used in the recent Macau World Series of Mahjong competition.
For riichi mahjong, the most common form of mahjong played in Japan, there were little information on the rules and strategy in English, but two recent websites are remedying this. Reach Mahjong Columns is a blog written by English-speakers to get more non-Japanese to play riichi mahjong. Two of the three contributors are young Americans who are professional mahjong players in Japan. There are some interesting tidbits about the professional mahjong leagues and tournaments in Japan, and plenty of information and tips on playing riichi mahjong. Yakitori Online is a new portal devoted to riichi mahjong, including a forum for mahjong enthusiasts to discuss strategy and other mahjong-related issues.
It does seem that the riichi scene world-wide is getting more exciting. There will be a European riichi mahjong tournament held next year, due to much interest from Dutch and Danish players. Too bad riichi mahjong is almost unknown in Singapore, save for arcade and computer games from Japan. It would be certainly difficult to get a proper Japanese mahjong set with the red dora tiles in Singapore!
Last but not least, for Singapore Style mahjong, there is a good Wikipedia article on Singaporean Mahjong scoring rules. All the well-known patterns and special hands are found here, as well as descriptions of special situations and danger scenarios.