Sort of.  Shanghai II and its shareware clone Gunshy are excellent games, but
they are not actually mahjong, although Shanghai does use mahjong tiles.
Rumor has it that there is a commercial version of japanese mahjong that
is only available in Japan.

There is a freeware Japanese-rules mahjong program called "Macjong 0.20a",
but it is reputedly buggy and incomplete.  It is available at umich in
<ftp://mirrors.aol.com/pub/mac/game/card/>.

There is also a shareware mahjong game under development that uses Chinese
rules.  It is reportedly pretty good and at least as stable as Macjong.
It's available at 
<ftp://ftp.luth.se/pub/mac/games/board/>.

More information about traditional mahjong is available at Jonathan's Mah 
Jong Page at <http://www.atdesk.com/jon/mahjong.html> and the Mah Jongg Home 
Page at <http://www.cs.utk.edu/~clay/mahjongg/>.