Recently I've been playing Kerbal Space Program. If you haven't heard about it yet it's this game where you build rocket ships and send them off into the great unknown. The thing that in unique about this game is that it has one of the most complicated physics engines I've seen in a game.
This game is very realistic, and because of this is also very complicated. In order to get in orbit of the earth is a challenge all it's self. In order for your ship to be able to escape the atmosphere it needs stages. The trick is to design a ship that has a good balance between power and weight, so you need engines in each stage that can handle the load of all the other stages fuel. This is where the game gets tough, in order to go to other planets, you need to find a way to get loads of fuel into space, but can't escape the atmosphere with all the fuel on one rocket.
At this point you set up space stations that hold fuel that can be docked to. These fuel check points aren't the only way to achieve interplanetary travel. You can also assemble your ship in-orbit by sending up separate parts and then attaching them together.
All in all it is a fun game that implements very realistic physics and is loads of fun. Give it a play if you can, you wont regret it.