One interesting feature is that you can make addons and create your own fictional star systems and planets, or download ones that people have already made. This program alone really changed how I thought about the vastness of space, zooming around the solar system and various stars. I would definitely recommend this to anyone interested in astronomy.
You can get it here (free of course):
http://www.shatters.net/celestia/
And here is the website that has all the addons that people have created for it:
http://www.celestiamotherlode.net/
To give you an example of what this program looks like, here are a couple planets I made:
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