I'm not sure exactly why it's been posted here (perhaps to kickstart a discussion about MUDs in general?), but The MUD Connector is a directory of MUDs/MOOs/MUS

I'm not sure exactly why it's been posted here (perhaps to kickstart a discussion about MUDs in general?), but The MUD Connector is a directory of MUDs/MOOs/MUSHes/etc. in operation, with descriptions, player reviews, activity stats, and so forth.