From the official website:
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For the aspiring MUD administrator with a vision, starting from a well developed codebase can be quite a hassle; stripping out all of the existing systems that conflict with your vision - like stock combat, magic, or currency - can be more trouble than it is worth. Likewise, starting right from scratch can be just as much of an onerous undertaking. NakedMud aims to let you start right from where you want to - a richly developed codebase without any game content.

NakedMud's goal is to provide a richly developed base containing no actual game content or mechanics. NakedMud also aims to be easily extendable, simple to set up, and full of utilities that will be useful to any development team, independent of the MUD they are working on. Such utilities range from those focused on designing game content (e.g. fully functional scripting language, OLC for building), to manipulating game content (e.g. object get/put/etc, character movement), and to programming new modules for your MUD (e.g. generic data structures, hook, event, and action handlers, a suite of tools for easy storage and retrieval from files).

Many thanks go to Brian Graversen for writing SocketMud, on which NakedMud was built. NakedMud is written in C, and uses Python for scripting. The latest version of NakedMud can be downloaded below. NakedMud is public domain; it costs no money to use, and comes with no licencing restrictions. NakedMud requires gcc >= v3.0.2 (other compilers untested) and Python >= v2.3. Startup instructions are contained in the docs folder of the download.
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Changes in 3.8.1:

VERSION 3.8.1 (Apr 07/09)
* newstyle messages with sock.send, ch.send, ch.sendaround, mud.send, and
mud.expand_text: can have embedded Python statements
* Triggers can now be run from Python ch/room/obj.do_trigs(type[, variables])