FlightGear Flight Simulator: free open-source multiplatform flight sim
Online War system for IL-2 Sturmovik and Iron Front in ArmA 2
Classic America's Army
A planetarium software that shows a 3D simulation of the night sky
Cosmos Tycoon is a open source transport simulation browser game.
tiny code, html webcam game or plug brain in each blob, java server
A server to exchange flightplan information between ATCs
The free and Open Source hockey simulator
SuperWikia: The Runtime/Realtime Environment for Codesmiths
Modular Minecraft server plugin - commands, events, ...
A open-source online football manager like hattrick