Kerbal Space Program is a rocketry simulator, in which the goal is to build a rocket, launch it, and (hopefully) recover the crew alive.
To quote the site:
In KSP, you must build a space-worthy craft, capable of flying it's crew out into space, without killing them. At your disposal is a collection of parts, which must be assembled to create a functional ship. Each part has it's own function and will affect the way a ship flies (or doesn't).
So strap yourself in, and get ready to try some Rocket Science! Currently the game is being built as an open sandbox. There are no objectives yet. You're free to build anything you can think of, and fly it wherever you want, even into orbit. The first versions are free to download and play, and will remain so forever. After the game reaches a certain state of development, it will be available for purchase at a discounted price, which will gradually increase up to it's final retail price as the game nears completion.
So by ordering early, you get the game for a lot less, and you'll get all future updates for free. My thoughts: At the moment this is fun to play with but the user interface for spaceship construction is very fiddly, probably not ideal for educational purposes. It's definitely worth keeping an eye on its development, since it would fit very nicely into any physics syllabus with a few improvements.