I had always interest in astronomy and I am very happy that I stumbled on this program I want to tell you about. It's called Celestia and is a free real-time space simulation, available for Windows, Linux, and Mac OS X. Celestia is Open Source and the ongoing development is made through voluntary contributions of individuals all around the world, may it be coding, texturing, modeling, or providing astronomical data.

Celestia is getting more and more popular and even the NASA is using it for teaching purposes. The reason for this is a simple interface combined with a powerful engine. Here's a transcript:

"Unlike most planetarium software, Celestia does not confine you to the surface of the Earth. You can travel throughout the solar system, to any of over 100,000 stars, or even beyond the galaxy. All travel in Celestia is seamless; the exponential zoom feature lets you explore space across a huge range of scales, from galaxy clusters down to spacecraft only a few meters across. A 'point-and-goto' interface makes it simple to navigate through the universe to the object you want to visit. Many add-ons and vastly improved textures of high resolution are available for Celestia."

If you visit the site you'll notice the mainpage isn't really up to date but the forum is quite active. I'd recommend to read the Celestia User Guide if you want to know details about the program.

I've put up some links to pictures, to give you a little preview of what Celestia is capable of:

#1 - North- and Southamerica with the exact light situation at around 21:11 UTC.
#2 - Close-up of the European Alps at a distance of about 1.635 km.
#3 - Mars, the red planet.
#4 - This view you'd get from a 22.656 lightyear distance of the sun with around 2.000.000 stars (there are of course a lot more stars in reality but so far the largest database available for Celestia only contains 2 million)

Edit: Links were removed, I deleted the pictures some time ago. There are lots of pictures to be found in the forums, though! :-)