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.
JamochaMUD: Java client with most standard features. Windows and Linux builds available.
KildClient: GTK+ client with a built-in Perl interpreter. It runs under Windows and Linux.
MudMagic: Gnome client for linux and windows with plugin support.
Papaya: GTK client with most standard features and plugins. Win32 and Linux builds available.
TinTin++: Console mud client, with most standard features. Windows, Mac OS X, and Linux builds available.
tkMOO-light: A unix/win/mac client geared toward MOO systems. Doesn't support ansi.
Trebuchet: TCL/Tk client for Linux and Windows. Supports MCP.