Add to the list if you know of others. I can also add some description to the sites if you have info on any aspect of those listed.

VDiplomacy: http://www.vdiplomacy.com/

WebDiplomacy: http://webdiplomacy.net/

PlayDiplomacy: http://www.playdiplomacy.com/

Backstabbr: http://www.backstabbr.com/

Diplomaticcorp: http://www.diplomaticcorp.com/

Bounced: http://www.dipbounced.com/

Diplomacy.ca: http://diplomacy.ca/vforum/index.php

Stabbeurfou: http://www.stabbeurfou.org/