Mcl

Console client, with most standard features. Allows Perl and Python scripting.
http://www.andreasen.org/mcl/index.shtml

SClient

GTK client, with most standard features.
http://home.swipnet.se/heathen/linux/sclient.htm

GNOME-Mud

GTK client, with most standard features. Allows Python scripting.
http://amcl.sourceforge.net/

RMC

GTK client, with basic features. Aims to be easy to use.
http://www.geocities.com/rmc_client/

Xpertmud

KDE client, with basic features and plugins. Allows Perl, Python and Ruby scripting.
http://xpertmud.sourceforge.net/

MudTelnet

Console client using the real telnet protocol. Allows triggers and scripting by perl, C, C++, and shell.
http://www.itechsc.com/software/free/mudtelnet/

Powwow

Console client with most standard features.
http://linuz.sns.it/~max/powwow/

Kmud

KDE client, with most standard features.
http://www.kmud.de/

TinyFugue

Console client, with most standard features. Has some MOO specific support.
http://tf.tcp.com/~hawkeye/tf/

Mmucl

Tcl/Tk client with a console version. Provides Tcl based scripting and basic features.
http://mmucl.sourceforge.net/

Ymusk

Tk mush client written in perl.
http://ymusk.sourceforge.net