FlightGear Flight Simulator: free open-source multiplatform flight sim
A very efficient tutor.
Chess Database and PGN viewer
A modern, cross-platform Quake game engine based on FitzQuake.
A cross-platform port of Hexen II game.
GemRB is a portable open-source implementation of the Infinity Engine
Android, UIQ 3.0, Series 60 Edition 5 open-source software
The ultimate cataloguer
Game engine supporting many 3D/2D formats and graphic effects
LGSL (Live Game for PHP 5.4-8+: online status for various gameservers
Online War system for IL-2 Sturmovik and Iron Front in ArmA 2
Full featured MUD server with all the trimmings.
JavaScript framework to build game engines, games, emulators and apps
tour based strategic game
General Purpose Engine for Graphic Adventures.
Framework for producing web-based database applications
Jeopardy in HTML and JavaScript
Java Settlers
NES/Famicom emulator based on FCE Ultra.
Teach your students how to figure out tricky texts in any language.
Defeat all the beasties to claim victory!