Kerbal Space Program: Let's Light This Candle
Man, I can't believe someone hasn't thought of this before. A game about building rockets and seeing if they'll fly (or crash spectacularly).
The premise is simple: build a rocket in the vehicle assembly building using various parts (You want a winglet on your solid rocket booster, you *add* a winglet. Hell, add two and see what it does), take said rocket out to the launch pad, light that candle, and see if you can keep it pointed up and your Muppet-like astronauts alive through splashdown. If you need to tweak your rocket, just go back to the vehicle assembly building, make your changes, and rinse and repeat.
Within a half hour I had a rocket that put John Glenn to shame, though my attempts to complete a full orbit were...much less successful (I blame the lack of winglets).
If you're feeling even a bit nostalgic over the last flight of the Space Shuttle going on, then do yourself a favor and download this free little gem.