GarouMUSH is one of the oldest World of Darkness MU*s, and almost certainly the oldest if continuous operation is a criterion — it opened in 1993, and has been going ever since. As one might guess from the name, it focuses almost entirely on Werewolf: the Apocalypse, with the occasional other critter popping up now and then.  I played there from about 2000-2003, returned for a few months in 2013, and as of 2015 am now back again... if you're reading this and not currently playing there, drop by and check it out. Felix needs more people to bug!