Linux and Unix

GnomeMud
GNOME client with most standard features. GNU General Public Licence.
http://live.gnome.org/GnomeMud

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

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

Mmucl
Tcl/Tk client with a console version. Provides Tcl based scripting and basic features.
http://mmucl.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://hoopajoo.net/projects/powwow.html

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

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

TinyFugue
Portable console client with most standard features. Has some MOO specific support.
http://tinyfugue.sourceforge.net

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

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