Acquiring
MUSHclient can be acquired for free from http://mushclient.com/mushclient/mushclient.htm
The most recent (perhaps not 'officially released') versions can be found here near the bottom of the list.
Other Information
Features, screenshots, etc. can be found here as well.
Personal Opinion
I've used this client for 6 to 7 years now, and it's been in development the entire time (Which is a good thing, as it's far from being a dead project).
Originally the Perl scripting support attracted me, as well as multiple worlds within the same window. The built in spell checker is particularly useful.
I've also written my own 'colornames' plugin for it that automatically colors everyones names a random color to make it easier to distinguish names from the rest of the text.
MUSHclient ColorNames Plugin
