This article has a nice list of 50 open source applications for science education.

Open Source: Astronomy and Space Exploration

1. Celestia

A perennial favorite, Celestia allows users to view the night skies from any point on earth at any time or to see how the sky would look from any point in the known universe. The interface is simple to use, and it uses real telescope imagery of stars, planets, asteroids, and other objects in space whenever possible. Operating System: Windows, Linux, OS X.

2. Stellarium

Stellarium is like having your own personal planetarium on your computer. In fact, it’s such an accurate depiction of the night sky seen with the naked eye or simple binoculars that it’s used by many planetariums. Operating System: Windows, Linux, OS X.

KStars

KStars offers the same functionality as Stellarium, plus some tools for amateur astronomers, like an observing list, FOV editor, altitude vs. time tool, and more. It comes with a default catalog of all stars up to magnitude 8, but you can expand it with a catalog of stars up to magnitude 16. Operating System: Windows, Linux.

4. PP3

An excellent tool for teachers, PP3 creates star charts that are detailed enough for use in PowerPoint presentations or books. Note that in order to use it, you will also need LaTeX. Operating System: Windows, Linux.

5. Cartes du Ciel (Skycharts)

Another option for creating star charts, Cartes du Ciel uses data from 16 different catalogs. Don’t let the French name scare you off—there’s plenty of English documentation. Operating System: Windows, Linux, OS X.

6. Open Luna

If looking at pictures of the sky isn’t good enough for you, Open Luna is an open-source project dedicated to sending actual spacecraft into space and eventually the moon. The group’s first mission is scheduled to launch next month. Operating System: N/A.

7. DevelopSpace

DevelopSpace has an even more ambitious goal—developing the necessary technology to enable a one-way trip to Mars in the near future. Students and others who get involved can contribute directly to solving the technical challenges posed by a mission to Mars. Operating System: N/A.

See the complete list.