ehMOO is a 3-D graphics extension to the text-based LambdaMOO 1.8.1 world server. It includes a 3-D graphics client (ehwt) as well as an alternative lightweight Java-based world server (miniMOO) and various Java-based editing tools.

http://ehmoo.sourceforge.net

First Person Shooters, Multi-User Dungeons (MUD)