FlightGear Flight Simulator: free open-source multiplatform flight sim
Online War system for IL-2 Sturmovik and Iron Front in ArmA 2
Opengate is a Jumpgate remake
A robocode championship manager made using JavaFx
A planetarium software that shows a 3D simulation of the night sky
Mad Science
The free and Open Source hockey simulator
BlueBox is an alife open source game.
A libgdx pinball game.
Modular Minecraft server plugin - commands, events, ...
A open-source online football manager like hattrick