Java

Bluemud
Project to rewrite the Varium MUD server in Java, incorporating a Python interpreter for world development.
http://bluemud.sourceforge.net/

JLMud
(DUMB- Diabolically Uncrashable Mud Builder) Java/Lisp Mud Environment, server implemented entirely in Java consisting of Lisp-like language interpreter specialized for the creation of MUDs.
http://sourceforge.net/projects/jlmud

JMud
MUD server written in Java(TM) from scratch. Portuguese (Brazilian) version available, Multi-lingual support in progress for Spanish and English.
https://sourceforge.net/projects/jmud/

JUDGE
Java Universal Distributed Game Engine - based on a client/server architecture and written entirely in java. This d20 based game engine will eventually make use of JINI to allow certain functions to be performed locally.
http://sourceforge.net/projects/judge/

Key
Boasting a complete telnet protocol implementation, support for object property reflection, a transparent storage management mechanism, and a secure advanced permission system.
http://key.sourceforge.net

WolfMUD
Intended to be simple to install and run. Contains GUI based World Builder.
http://www.wolfmud.org/