Here is possibly one of my favorite games ever... Yet almost no one knows about it. X3 I got it back when it was only 15 US dollars, now the price has gone up to 20-something.
What is Kerbal Space Program? Well, this video should give a slight hint:
https://www.youtube.com/watch?v=LOC9UV7HfkQDo note that most of this is older footage and this game has many updates.
It is also extremely modifiable, there are LOTS of addons for KSP, including things like a project orion (
http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)) mod, and even things that allow you to change the positions of planets! (That mod is specifically called Hyperedit in case you're wondering)... There are a few people who are working on multiplayer mods, because the game itself is singleplayer, but none of those have come out yet, and also there was an additional planet mod which was being worked on, I'm not sure if it still is...
Further videos which are neat, and that you should watch:
http://www.youtube.com/watch?v=RkDOOsGg-9Ihttp://www.youtube.com/watch?v=srggpVC7qgE