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 planetarium software that shows a 3D simulation of the night sky
3D urban environment renderer : browser-app ; as well as a GTA clone
Mad Science
The free and Open Source hockey simulator
A open-source online football manager like hattrick