Kerbal Space Program is a rocket/plane building game where you send little green men(and now women!) to space. Your goal is to design, build, and launch your own spacecraft or airplanes. We've got rockets, fuel tanks, solar panels, powered wheels, heatshields, parachutes, command pods, and a lot more.
There are three modes:
Career, where you collect Science to unlock parts and complete missions to collect funds that will help you build your spacecrafts and work to build your little startup space company into the space program of an entire world.
Science mode, which is just like career, but all you have to worry about is science, and no funds.
and Sandbox, where you have free reign to create all the spacey(or otherwise) creations your heart desires!
In this update we've recieved female Kerbals, a new aerodynamics mode, possible overheating of parts(deadly re-entry), fairings, resource scanning and drilling, and more!
This game has an incredibly active modding community, letting you add things like attachment ropes, autopilots, new parts, lasers, weapons, motorized parts, and even new planets and solar systems!