zMUD: This is a very old (and free) version
of "Zugg's MUD Client".
It's good, for an old Win32 program.
gMUD:
Somewhat less sophisticated than zMUD, but some people like it.UNIX MUD clients: Don't download these unless you run UNIX and know how to compile from source code. Presumably these will also work on OS X, if you can compile on that.
gMOO: older version of Gert Scholten's program. Triggers work well, and it's an excellent player client, not so great for coding.
gmoo: A more recent version of Gert Scholten's program, apparently taken over by other developers. Triggers are kind of broken, but easier to write code with than the older version.