Stellarium is a cross platform planetarium application which is capable of rendering 3D photo-realistic skies in real time. This means that what you see on your screen will be what you will be able to see with the naked eye, or with a telescope. This is a fantastic way of learning what is what in the night sky, as it displays stars, constellations, planets and nebulas.

Stellarium's Night Sky
Image From: http://www.freesmug.org
To install Stellarium:

sudo aptitude install stellarium

Once installed (May take a while, as it is quite a big application) you can find it under the Educational section within your main menu (or by running the command stellarium in the terminal).

If you are having problems running the application, this may be due to some problems with the resolution that the application is running at. You can change the default resolution by running the following command:

gedit ~/.stellarium/config.ini

and changing the following lines to the correct screen resolution for your computer:

screen_w = 1024
screen_h = 768