[Rapscallion] is a popular Macintosh client. I noticed that both the client and the source code have been released for "free" use in March 2005.
[Savitar] is a software application designed to connect people using Macintosh computers to MUD/MUSH/MOO/MUVE servers. Savitar is available for both OS 9 and OS X. Last updated in April 2005.
A very simple mud client, not much more than telnet with a bar to type that is not part of the output window. This is probably better for use with Classic Mac OS, especially pre-Mac OS 8 as it's simple to use and probably consumes considerably less memory than those noted above. It is not in active development as far as I know and can be obtained via a link on the page mentioned below under Other Clients.
[The Mac OS MUD Zone: Clients] Someone might add any of these to the list here with their own heading that are in active development or particularly good.