Having once again been sucked into promising to write some code for a MUD (Sanity's Edge, my third or fourth iteration as a coder there), I had to find a MUD client for MacOS. Some googling led me to The Mac OS MUD Zone and thence to Mud Walker, which I'm currently trying out. Given that I'm never been able to use any GUI client on a platform other than Windows (and even then, only out of necessity), it's a bit of a shift. I can't complain much, it connects, it has a separate window for input, which actually is a bit weird. I'm used to a line at the bottom of the main window with an input line there, separated by a status line - something like ircII or irssi's interface. The default view has no status line at all, and has one of the smallest default fonts (Monaco 9.0) I've seen in an application. All in all, fairly stable, although I'll see how bad it gets when I need to use ed.
(Edit 0130h: no command history that I could find and no ability to start/stop logging made me grumbly. So I tried Atlantis, which looked ok until I started using it. It defaults to spellchecking input area on, the word "wiz" isn't in its vocabulary, and no command history. Doesn't anybody do that any more?)