Odamex is a free, cross-platform, modern client-server Doom engine
LGSL (Live Game for PHP 5.4-8+: online status for various gameservers
DOOM / Heretic / Hexen source port with enhanced graphics and UI
Web based Game server control panel
The famous Maze War game recreated as a modern web application.
Cross platform administration tool for Source Engine dedicated servers
GraphOX is awesome
Cube2:Sauerbraten Mod with various features
Tools for installing and setting up dedicated Battlefield servers
A multiplayer First-Person-Shooter game built in Java 3D