Update (If I am missing any, please let me know)
Web based:
www.playdiplomacy.com: 166,608
webdiplomacy.com: 200,015
www.phpdiplomacy.net: 473,341
www.worldleadersthegame.com: 562,956
www.stabbeurfou.org: 3,332,599
www.dipbounced.com: 6,689,015
Web/e-mail interfaces:
www.floc.net 1,501,924
diplomacy.ca diplomacy.ca 8,091,084
www.diplomaticcorp.com 13,803,643