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 Excellent construction sandbox for building and launching rockets
KSP at its core is a rocket building and launching sandbox. When creating a rocket you can place engines, fuel cells, structural elements, and even parachutes, allowing you to build a rocket in any shape and size. The end result can even look like a giant doughnut, which might be difficult to get it into outer space. Even if you fail, the entirety of the process is really enjoyable.
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.
Con Steep learning curve
Kerbal Space Program is not really a pick up and play title. It can take hours just go grasp the basics and the game will keep throwing new concepts at you as you progress into space, even stuff like orbital dynamics. Because of this, it can easily take dozens of hours to truly master this game.