I played Achaea for a while....Best one I ever saw...Don't even know if it's in the Diku Mud family, it's mostly custom coding...I still logon sometimes to see what's up...
http://www.achaea.com I lost so many hours there...Still pretty active tough...always 200+ players online...mostly a roleplaying mode, there's a shitloads of quests etc...and the world is HUGE and it's all custom areas...