Stellarium – An Open-Source Planetarium

Stellarium is an open-source astronomy application for all platforms. In their words:

Stellarium is a free open source planetarium for your computer. It shows a realistic sky in 3D, just like what you see with the naked eye, binoculars or a telescope.
It is being used in planetarium projectors. Just set your coordinates and go.

This makes for a great alternative to expensive telescopes when learning about space in school. As it is open source it can be installed on all machines in a suite, or across a school for both free access and as a teaching resource.