http://kerbalspaceprogram.com/
Came across this little game the other week, still in development but rather fun.
"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!"
Still havent managed to get a rocket to the moon, but thats the fun of a realistic physics engine where thrust and trajectory all matter.
