The Endless Forest
"You are a deer. So are the other players. You meet each other in an endless forest on the internet. The setting is idyllic, the atmosphere peaceful. You communicate with one another through sounds and body language."
Neopets Classic
Relive the nostalgic era of Neopets! Registrations open every few months.
vgperson
A listing of games (mostly rpgmaker) translated by vgperson. I recommend Ib and Wadanohara and the Great Blue Sea!
Rate My Server
A listing of private servers for the MMO Ragnarok Online!
Baroque
English translation of Baroque for the Sega Saturn - which according to fans is the version to play! You'll need an emulator!
Spellborn
Fan revivial of the tragically short-lived, original action-combat MMORPG: The Chronicles of Spellborn. If you've any interest at all in exploring (nearly) abandoned MMOs, this one is my favorite. The atmosphere is incredible and the gameplay still holds up!