SEGA MJ-NET is a fun little Mahjong client that you can play on PC and mobile devices (and arcades, too, if you live in Japan). It has a lot of fluff features, such as cosmetic customization, flashy effects, minigames, and regular event rotations. While the game features many premium aspects, free players can get most of the experience all the same, and private games are also possible (and free!). It won't replace Tenhou for you, most likely, but it's a solid and enjoyable client that reminds us all of a certain Mahjong client before it went down the dark path.
Considering the entire program and interface is in Japanese, this guide was created to help English speaking players who are interested in giving it a go.