Kerbal Space Program
Anyone here plays it?
It is a really cool game where you take care of your own space program, building rockets and piloting them, landing on planets, docking modules and creating a space station over your planet, or just messing with the green aliens that are inside the rockets.
It has a steep learning curve, but it can be used to learn a lot of orbital mechanics.
Currently there are two gamemodes, Sandbox and Career. In Sandbox, you have all the parts at the start, but in Career you must do experiments to collect science and unlock new parts.
It is still in beta, but there aren't a lot of gamebreaking bugs, most of them caused by using more parts in a rocket than you need. It receives regularly (the newest one added asteroids) and it is $26.99 (also on Steam)