ErlyMUD is a rather minimalistic MUD server, written in Erlang and making use of the excellent OTP libraries. The aim is to have solid support for exploration and roleplaying, within a highly fault-tolerant environment where system crashes or reboots are more of an exotic curiosity than a commonplace thing.
DevClient is a highly customizable MUD client designed to be friendly and easy to use. It features full parsing of ANSI color codes, support for aliases, macro binding, keypad navigation, MCCP 1 and 2, accounts and automatic login management, prompt parsing, logging, and more. Its functionality and graphics can be extended with simple server definition files. It can be easily translated into any language. It can detect when a new version of itself is available and upgrade itself without restarting.
php mudnames is a PHP class that can generate random names for characters used in role playing games such as multi-user dungeon (MUD) games. Although the names do not mean anything, they sound very much like typical names of characters that you often see in this kind of game. It can retrieve text data from one of several dictionary files. The dictionary data is used to generate random names based on selected capabilities and used particles.
libtelnet is a small library for writing telnet protocol handlers in C. It uses a fully correct state machine model and full RFC 1143 (Q Method) option negotiation. It includes optional support for MUD-specific protocol extensions such as MCCP2 and ZMP. Several test programs are included, such as the very useful telnet-proxy utility.
Zombie Nation MUD (ZNMud) is a Multi-User Dungeon engine. It is based on the Teensey engine for its lower level socket handling. It was designed to play out zombie outbreak ideas but can easily be expanded to host other concepts. It's a massive multi-player text-based game (although it does support MXP). It also supports modern day weapons, etc.
MV3D is a virtual world and multi-player game framework for use with Python. It was designed with scalability in mind and is able to distribute a world across as many servers as needed while dynamically balancing the load. The simulation framework is not specifically slanted towards any one genre of online game or virtual world, and can just as easily be used for a space game as a fantasy setting. Objects on an MV3D server can be simulated using the ODE physics engine for realistic interactions. A single server is able to host thousands of of simulated objects. The client works with both the Ogre3D and Panda3D renderers.