Hello,

We've recently launched http://www.jadedragonmahjong.com/

Its free and you can play against human or computer players.

It currently has;
Chinese Official rules
American rules
Taiwan 16 tile
Hong kong rules / Chinese old style
European rules

American rules will be finished by the end of May and we'll be making
continued improvements as well as offering a downloadable version
fairly 
soon.

best wishes

Jade Dragon Mahjong