Windows
AL Client, AvPlay, DDEmud, Dragonsong, Fire Client, GosClient, MuckClient, Mud Master, Mudism, MUSHclient
Clio Mud Client
A text-based MUD2 client for Windows, Unix, Linux, and MacOS X. Has many features.
eMUDs
client written in EMACS with LISP scripting support. Runs on Linux and Windows.
GGMud
GTK client with most standard features. Win32, OS X, and Linux builds available.
MudMagic Mud Client
Gnome client for linux and windows with plugin support.
Papaya
GTK client with most standard features and plugins. Win32 and Linux builds available.
tkMOO-light
A unix/win/mac client geared toward MOO systems. Doesn't support ansi.
Trebuchet
TCL/Tk mu* client for Linux and Windows which supports MCP.