GNOME-Mud
GTK client, with most standard features. Allows Python scripting.
Mcl
Console client, with most standard features. Allows Perl and Python scripting.
Mmucl
Tcl/Tk client with a console version. Provides Tcl based scripting and basic features.
MudTelnet
Console client using the real telnet protocol. Allows triggers and scripting by perl, C, C++, and shell.
TinyFugue
Console client, with most standard features. Has some MOO specific support.
Xpertmud
KDE client, with basic features and plugins. Allows Perl, Python and Ruby scripting.