Gypsum is a MUD client built with Threshold RPG in mind. It has many features that were requested or refined by Threshold's players and admins, and is actively maintained by a player (Rosuav). If you've used RosMud, this is the spiritual successor to it and should feel very comfortable; if you've used Gmud, the basic look-and-feel should be familiar to you. Gypsum supports multiple simultaneous connections (to the same server, or to different servers), infinite scrollback, timestamping of received text, and extensive customizability. It is fully open source (MIT-licensed) and cross-platform (tested on Linux, Mac OS, and Windows).
If your Gypsum is stuck in password mode, type this on the input line:
/x subw->passwordmode = 0
Deprecation on Windows
Due to the fact that this client is no longer able to be updated on Windows and until such time as it will be, it is recommended to use clients which are, such as BeipMU or Mudlet.