Well I recently bought Kerbal Space Program on Steam sale for $15 and I have to say it's one of the most addicting games I've ever played (and it's only in Alpha!) I believe there's a demo on Steam if any one who hasn't played it wants to try it out.

If you've never heard of it before, basically what you do is build spaceships, rovers, space stations, etc. and travel all over the universe. When you build an object, you launch it from the planet Kerbin. Kerbin is extremely similar to the Earth and has a moon orbiting it called (none other than) "Mun." There are also other planets you can land on and there's no limit to what you can create. If you want to launch a missle with a seat on it and crash your little alien into Kerbin's soil, you can. Or you can challenge yourself by trying to land on the Mun or any of the other planets. After about 20 hours of gameplay in just a few sittings, I've only landed on the Mun once :P. However, every time you launch your own creation and break through the atmosphere, you instantly want to do whatever you can to keep the ship in space for as long as you can. The replayability is extremely high and there's even a cockpit view to look at the mission from the terrified little space men that are almost destined to be sacrificed :P.

The game right now has around 5 small missions, a sandbox mode, and a career mode is on the way. I hope you enjoy it as much as I am and there are plenty of youtube videos on how to make a successful trip. Also, I believe the game's standard price is $20 on Steam if you were considering a purchase.

PS - The game does have many graphic/video setting under the options in case you're not sure if your PC can run it. I run it fine on a laptop so average PC's should be able to run it without any trouble.