Universe Sandbox is exactly what its name implies: a place for you to play around with astronomical bodies and see how changing various things will effect the macro-universe. I think it is one of the most unique and awesome titles on Steam. It is well put together, the physics in the game accurately demonstrate real-life laws, and the recent update that puts you directly into a tutorial the first time you start the game really helps you figure out the, at first intimidating amount of, options that you can play around with.
Whether for work, play, or educational purposes, Universe Sandbox is an excellent title that I don't think any Steam account should lack. If you have *any* interest, even a minute amount, in astronomy, physics, or even if you ever wondered what would happen to Saturn's rings if Neptune flew too close, you need to have this game. It's very relaxing to play with, and the the engine is extremely deep - allowing you to truly let your imagination run wild.
The GUI is clean and efficient, and the pre-built simulations and environments are extremely well put together and fun to play with. Some examples include:
Our Solar System
Earth & Moon
Saturn & Moons
Small Scale Gravity
Buy now, build your own galaxy or satellite system, and watch what happens as time passes and you introduce variables into the equation.
*Bonus:* You can smash an asteroid into Earth to see what would happen, awesome for those bad days at the office.