Mac users have a few options for clients. There is a new client currently in beta called Atlantis with the goal to create an experience more like SimpleMU for Mac users. The standard choices are currently Savitar, Cantrip, and Rapscallion.
Atlantis
Atlantis is a new client for Mac that is currently in beta testing. It aims to provide a number of features, including logging, activity notices, spawns, hotkeys, an address book, a windowed system, an event system, and a highly customizable interface.
It is in active development and the beta version is currently available for free download. It may become shareware once it has reached full release. You can read more about the developer's plans here.
You can find out more about Atlantis here: http://www.riverdark.net/atlantis/
Savitar
Savitar is another client for Mac. It creates a separate window for each world connected.
It will run under both OS 9 and OS X. It features customizable window colours, speech output, triggers, macro keys, drag and drop, gags, and more.
It is shareware. A single-user license is $15, though it may be used for free. (Any idea about nag screen, limited features, etc.?)
You can find out more about Savitar here: http://www.heynow.com/Savitar/index.html
Cantrip
Cantrip is primarily a MUD client, but can be used to connect to a MUSH.
(Needs expanding.)
I believe Cantrip is freeware.
You can find out more about Cantrip here: http://www.solidsun.com/cm/
Rapscallion
Rapscallion is also primarily a MUD client. I believe it is no longer in active development.
(Needs expanding and confirmation.)
I believe Rapscallion is freeware.
You can find out more about Rapscallion here: http://www.geocities.com/SiliconValley/Way/6818/
TinyFugue
TinyFugue is designed to run on any flavour of UNIX. TF should compile 'right out of the box' on OS X if you have Developer Tools installed. It is rumoured to have something of a learning curve to it.
I believe TinyFugue is freeware.
You can find out more about TinyFugue here: http://tf.tcp.com/