GGMud
GTK client with most standard features. Win32, OS X, and Linux builds available.

KildClient
GTK+ client with a built-in Perl interpreter. It runs under Windows and Linux.

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.

TinTin++
Console client, with most standard features. Win32, 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.