Kerbal Space Program is an indie game about building rockets that get the cute little Kerbals into space (and hopefully back, but probably not).
Pro It grabs hold of your attention and never lets go
Build rockets, space planes, space stations, rovers, landers, probes. Explore planets, moons, capture asteroids. With the addition of mods you can even test warp drive, set up a communications network, colonize a planet, mine for resources and much much more. It can even simulate our solar system (the in-game one is very similar, although smaller to encourage new players.
Learn orbital mechanics. Inspire yourself to learn about space and space travel history of human. This game is not only fun - it is inspiring.
Pro Teaches orbital dynamics & rocket design fundamentals in an accessible manner
The physics engine does a reasonably accurate job of replicating the fundamentals behind both orbital dynamics and rocket design, so by going through the tutorials and experimenting, you're actually learning some real rocket science.