Here are some other forums:

Chopinmusic (Chopin related):
http://www.chopinmusic.net/forum/index.php

Piano World Forums (Biggest piano forum in the world):
http://www.pianoworld.com/ubb/cgi-bin/ultimatebb.cgi

Gamingforce Forums (recording and sheet music exchange):
http://www.gamingforce.com/forums/

Piano Street Forums (collection of Sheet music, you have to pay for it though):
http://www.pianostreet.com/smf/index.php

Well-Tempered Forum [on Eve] (Off-topic discussion with musicians):
http://well-temperedforum.groupee.net/groupee

Rachmaninov Society Forums (Small forum, Rachmaninov discussion):
http://www.rachmaninoff.org/forum/index.php

Dasdc Forums [Da Speed Demon Community] (Strange place, but it really has to do with piano ):
http://www.dasdc.net/forum/index.php

Pianophilia (About piano sheets, chamber music and sound recordings):
http://www.pianophilia.com/

Mozart Forum (The name speaks for itself):
http://www.mozartforum.com/

Pianomajeur (in French ):
http://www.pianomajeur.net/

I would like to add one site:

International Society Of Collegiate Performers http://iscp.excell.org

I know this guy. Pretty funny but tiring in the long run. He collects any performance played with fury and have lists of the fastest performances of piano pieces. Pretty crazy and he was posting at the old forum as well in the beginning. Seems like he is running a successful forum as he has a LOT topics and posts. They got their own languages as well.

Know a lot of the others as well and am registrated in at least half of them.