Stellarium is a free astronomy software. You can download, use and distribute it under GPL license. It has been very popular. Since it was launched, more than 18 million copies have been downloaded.
Stellarium displays the sky in a realistic 3D view, similar to what you see with your naked eyes, through a pair of binoculars or through a telescope. The view can be zoomed in or out to give the desired level of details. It has a built in catalog of stars that contains more than 600,000 stars. Additional catalogs can be downloaded to make the total number of stars exceeds 200 millions.
In addition to stars, Stellarium displays solar system bodies, deep sky objects like galaxies, nebulae and star clusters. Artificial satellites can also displayed using the included Satellites plugin.
You can navigate different parts of the sky by using mouse buttons or cursor keys. You can move up and down, left or right. If you disable atmospheric effects, stars will be shown even during day time. When you hide ground, objects below horizon can be viewed.
Time can be set to real time or some time in the future or the past. This will make it possible to see past or future events like eclipses, conjunctions, occulations or even recent passes of artificial satellites.
Time speed can be real time or can be paused. The rate can be increased to be faster or slower. It can be forward or backward. When time rate is fast enough, stars and other objects apparent movement in the sky is more pronounced.
Stellarium serves also as a visual repository of cultures star lore. It shows how different cultures visualized the sky and what names they gave the stars. Currently, there are 14 sky cultures presented in Stellarium.
Stellarium is a flexible easy to customize software. You can add your own sky cultures, observer landscapes, plugins and you can write scripts for educational purposes or to perform complex or repetitive tasks.