Stellarium is open source (and therefore free) software that renders a very realistic view of the night sky in 3D.

The precision in Stellarium is impressive, and there is a large amount of interactivity. Enter some co-ordinates and get transported direct to the location, watch constellation lines being drawn on-screen, view cardinal points, simulate an atmosphere, and much more.

There are some great images of nebulae, a realistic Milky Way, and the full collection of all planets and their moons.

The software is also very customisable, allowing you to alter elements like the amount of twinkle in the stars, and you can add deep sky objects of your own.

You'll need to view Stellarium in a dark room for realistic rendering as details like the Milky Way or star twinkling won't be seen in a bright room.

Note: To download Stellarium for Mac OSX click here; and for the Linux version click here.