Here is a list of the software packages I recommend to people. They cover the basics all the way up to advanced astronomy and a few others for good measure. All run on Linux, Mac and Windows and all are free!
Stellarium is a planetarium for your computer. It shows a realistic sky in 3D, just like what you see with the naked eye, binoculars or a telescope. Great for presentations and learning the sky, a good one for beginners.
A space simulation program that lets you explore our universe in three dimensions.
Great star charting tool for the more advanced observer!