A planetarium software that shows a 3D simulation of the night sky
Stellarium renders 3D photo-realistic skies in real time with OpenGL. It displays stars, constellations, planets, nebulae and others things like ground, landscape, atmosphere, etc.
A free transportation simulator game
Simutrans is a cross-platform simulation game where players try to successfully manage transportation systems between places by land, air, and water for passengers, mail, and goods. Planes, ships, trains, trams, trucks, buses, or monorails are at your disposal, but factories operate based on contracts and passengers can only travel to their set destinations.
A 3D simulator game where you can drive, fly, and sail
Rigs of Rods is a 3D simulator game where you can drive, fly and sail various vehicles using an accurate and unique soft-body physics engine. Since 2014, the development continued at https://github.com/RigsOfRods/rigs-of-rods UPDATE: 10/30/2016: removed opencandy installers, replaced with zip files.
Online War system for IL-2 Sturmovik
SEOW is a dynamic multiplayer campaign system for running on-line wars using the IL-2 combat simulator. SEOW is a free add-on for IL-2 under the GNU Public Licence. For an install of SEOW, you need the MP, the DB and the DCS, 3 downloads in total.