Warzone2100, a real time 3D strategy game. 100% open source and available on nearly any OS you can think of. This is a full game, with an extensive single player campaign and fully integrated multiplayer. Been playing this for 10+ years now, never gets old. It was one of the first 3D RTS games of its time, and has a very unique style of gameplay. Rather than have a limited number of pre-configured units and support structures. Warzone instead starts the player out with next to nothing, and the only way to advance is through research of technology. The game puts a lot of emphasis on artillery, air support, and unit design. Design your tanks and aircraft as you see fit in real time, choose the weapons, armour, and propulsion to fight your way to victory. Orbiter Space Flight Simulator, a space flight simulator. Similar to KSP, but at a much more serious and in depth level. Takes place in the solar system rather than a fictional planet. Love this game. While KSP takes a more relaxed and arcade like approach to space flight, Orbiter does the opposite. It\'s numerous navigation tools and movement modes will quickly overwhelm any new player hoping to jump into the cockpit and press the gas button to go into orbit. If KSP doesn\'t fit your space flight bill, definitely give this one a shot.