TeensyMud is a very simple mud server written in Ruby. It uses a multiplexed single-threaded network driver using Acceptor/Reactor and Observer patterns and YAML formatted text files as a database. It is also a pure event driven system where all game objects communicate with each other via events.
This server was initially created in response to an Aprils Fool's joke posted by Erwin Andreasen on TheMudConnector? forum announcing a 1K mud contest. Those obfuscated versions can be found at the ftp site under the names t1k1.tgz to t1k4.tgz.
Ruby RDoc generated html documentation is available. For questions about TeensyMud, I've written the TeensyMudFaq. There are also a number of pages here on the wiki documenting pieces of the server and posts in the forum.
The latest stable release can be downloaded from the ftp site or checked out of the subversion repository.