Kerbal Space Program, found at http://kerbalspaceprogram.com/ is currently an indie game in alpha development, in which you run a fictional space agency. You build, launch, and pilot your very own rockets. The game realistically models gravity, inertia, thrust, torque and air resistance (Though rockets re-entering the atmosphere don't burn up yet, its planned for later versions however) It's free to DL and play.
Here's a few videos to showcase some of the most fun moments:
A zany failure:
Another zany failure::
Here's a successful launch, with insertion into Kearth orbit, to show that the game isn't just Crashes: the Simulation: