Kerbal Space Program is a game where you build your own rockets, Then try to fly them. The game is currently in development, But you can purchase it and play the most recent version (0.17)
The Demo version is 0.13, It is very outdated.
The demo consists of only a Moon, One planet, a few parts, and that is about it.
The full version (18$) has other planets, more parts, Persistance, (Launch a satellite, Exit game, Is still there) Spaceplane Hangar, EVA (Space-walks), and a lot more features and fixes.
MODS The game has ALOT of mods available, From small rocket engines, To carts and rovers.
Unfortunately, Most mods are typically only for the full version, Although some are for the demo.
VIDEOS http://www.youtube.com/watch?v=CyTxsQhyqMA&feature=plcp http://www.youtube.com/watch?v=Kkeb5-u3-lY