Windows users have the largest selection for clients. The two main choices are SimpleMU and MUSHclient.
SimpleMU
SimpleMU is a very robust MU* client for Windows. It supports a number of features, including logging, activity notices, gags, triggers, events, highlighting, spawns, and macros. It is actively supported.
It is shareware; a basic version of the client may be used for free, or for a $20 USD charge, users may access the full suite of features. Future upgrades are then free.
You can learn more about SimpleMU here: http://simplemu.onlineroleplay.com/
MUSHclient
MUSHclient is a 32-bit client written in C++. It claims to be the fastest MUD client for Windows. It features a quick connect options, aliases, triggers, timers, plugins, and more. It is actively supported.
It is shareware. Non-registered versions have full access to all features, but there is a brief delay when the client is started up (a nag screen). Registration is $20 and future upgrades are free.
You can learn more about MUSHclient here: http://www.gammon.com.au/mushclient/