SClient - Small GTK-based MUD client for X.

Tintin++ - Text-based MUD client with read-line support. Actions, aliases, and substitutions. Quite portable.

Papaya - GTK-based mud client for Linux, Solaris and Windows. Amongst other things it features aliases, triggers, ANSI colour, plugins and multiple connections.

Kmud - Free mud client for the KDE Desktop under Linux/UNIX.

AMCL - A simple Mud CLient - AMCL is Mud Client for X-Windows written in C, using the GTK+ widget set.

RMC - Ron's MUD Client - A simple and easy to use MUD-Client written using the GTK.

Bmud - MUD-Client for GTK and Gnome.