FlightGear Flight Simulator: free open-source multiplatform flight sim
LGSL (Live Game for PHP 5.4-8+: online status for various gameservers
The ultimate cataloguer
Arianne is an engine to develop multiplayer online games like Stendhal
Online War system for IL-2 Sturmovik and Iron Front in ArmA 2
Analysis of Oware (Abapa) games
Framework for producing web-based database applications
Full featured MUD server with all the trimmings.
JavaScript framework to build game engines, games, emulators and apps
Java Settlers
Teach your students how to figure out tricky texts in any language.
Playing Go with web-based turn-based Go server
oGame-like browser space realtime strategy game based on xNova
Flash Quiz TV (A quiz game for all family)
A planetarium software that shows a 3D simulation of the night sky
Magic (the Gathering) Card Collection