These clients vary but all of them include at least some of the following features:

Separate areas for input and output.

The ability to edit your poses (I believe some even have spellcheckers!)

The ability to recall past input (This is great if someone misses your page or you need to repose.)

Logging, and in some cases the ability to automatically keep certain messages out of the log.

The ability to be connected to multiple MUSHes and/or characters at the same time.

The ability to pause the output. (This is good for very crowded or spammy RP.)

Saved information for your MUSHes and characters so you can give a command and it connects for you, including logging into your character automatically.

Programs

There are a number of clients out there to choose from, some require a fee after a trial evaluation period. We have listed here some of the more common ones, but this is by no means and exhaustive list.

UNIX AND OS/2

TinyFugue - http://tf.tcp.com/~hawkeye/tf/

MACINTOSH

Rapscallion - http://www.rapscallion.co.uk/
Savitar - http://www.heynow.com/Savitar/

PC

MUSHClient - www.gammon.com.au/mushclient/mushclient.htm
SimpleMU - http://simplemu.onlineroleplay.com