Trebuchet Tk is a client program for connecting to TinyMU* and MOO style social MUD servers on the Internet. It'll also work with combat oriented MUDs like LPMUD and Diku, but it's not targeted at them.
Trebuchet is designed to be a GUI MUD client that is both easy to use, and as powerful as the text based TinyFugue client. It also supports the MCP-GUI server-side dialog protocols.
I wish I'd found this client sooner. I've been stuck using Cantrip for years, and though it's a noble effort, it's pretty lacking in a few important areas. Trebuchet is a huge improvement, and though it has a couple of glitches, it's a powerful MU* client.