Hellraiser
Arcane
Damn Hellrasier, seeing all those screenshots of Kerbal seriously makes me want to try it out, it looks so much fun.
There's a demo on the website, download that and try it. Gameplay for the moment is mostly what it is in the demo, screwing around with making (usually) spacecraft made out of whatever combination of parts you pick (and that isn't too complex to make your PC lag, much), occasionally you plant a flag somewhere. The difference with the full game is that there are a lot more parts (most importantly rover wheels, docking ports, probe cores, electrical generators and spaceplane parts) plus a bunch of planets and moons (rather than just the Mun and Kerbin), also you can use mods in it. An actual reason to build and fly rockets (read: career mode where you manage a space program) is supposed to come soon(tm) in the next few updates, they're working on adding astronaut training in the next update for now.