Note: this is my custom client. The official BatMUD client can be found here.
Chiroptera is a MUD client written in C# and uses IronPython as a scripting language. It runs on Windows and on Linux. Windows version has a graphical user interface. Linux version has a text interface with 256 color support.
The client itself is quite simple, but it offers services to python environment which can be used to implement pretty much anything from triggers to separate windows.
Windows version requires .Net Framework 2.0
Linux version requires Mono.
(Binary download not yet available)
You can get the sources from http://gitorious.org/tomba/chiroptera
You need .NET Framework 2.0 SDK to compile on windows (or visual studio).
You need Mono 1.9 to compile on Linux.