I recently stumbled upon the Asian game Mahjong and came to like it. I already play Go and Chess online, but had difficulties finding a good, English-language Mahjong server.
Are there any English servers, and if so what are they?
You could also try MahjongExpress. I've never played on there, but it looks like it requires you to install software, kind of like the online poker games.
A friend notified me of a site he occasionally plays on: http://tenhou.net/
According to him, it's a nice site. Problem is that it's entirely in Japanese. He's lucky enough to understand Japanese but most people don't so I added this answer just for reference.
Update: I found a nice site where you can play (only) against the computer. English language, no setup, completely free and easy to use - a great introduction into Mahjong.
Update 2: This page lists several clients. I did not try them yet.
I've also fell in love with this game recently. To play against players I am trying out MahjongTime, they give you a certain number of chips to play a day and you can play off of that. (four times a day if you don't win any games) and play for 0 mp
They have different styles of play, Hong kong, Taiwanese, American... etc.
And different rules. Not sure I would trust them for cash games but it is a good relief for quick games.
I don't find it very social though.
The National Mah Jongg League has an "official" version of American Mah Jong online with a PC based client. It cost an additional $40/year to your membership/card for the league. I have never tried it but it does allow for live online play with the current year's card.