SharpMud is a MUD server Framework written for .NET 2.0 in 100% C#. Provides an applcation layer that allows for MUD developers to focus on gameplay, not server mechanics. See the website for more information, including project status and direction.
YaCS - A window based chatserver with the ability to log or kick users. You can connect to the server by using PuTTY or Microsoft Telnet, because the chat supports VT100 and the telnet protocol.