I just bought Kerbal Space Program off of the Steam Summer sale and am already loving it. It's got that "beauty in chaos" feeling that many of the great simulation games have perfected. Creating a monstrosity of a spaceship while crossing your fingers and hoping that it just launches is both hilarious and engrossing. Plus, I love the character design so much; these guys are adorable as they smash into the Earth's Kerbin's surface.
The newest update to the game is the first step of many that works towards the Career Mode of the game. You can now hire specific crew members and assign them to specific parts of the ship, which will likely become the ground layer of the bigger "mission" process of the Career Mode. I, for one, can't wait for a Career Mode, since adding some direction to a game like this will certainly help keep new players on track.
The update also includes terrain updates, a better Space Center, and much more. The full changelog can be read here.