Stellarium is a free, Open Source planetarium for your computer that shows a realistic sky in stunning 3D. Also available as a Portable version.

You see what you would see with the naked eye, binoculars, or a telescope, which allows it to be used in planetarium projectors. Just set your coordinates and blast off to the stars.

Stellarium Features:

Sky:

Default catalog of over 600,000 stars

Extra catalogs with more than 210 million stars

Asterisms and illustrations of the constellations

Images of nebulae (full Messier catalog)

Realistic Milky Way

Very realistic atmosphere, sunrise and sunset

The planets and their satellites

Interface:

A powerful zoom

Time control

Multilingual interface

Scripting to record and play your own shows

Fisheye projection for planetarium domes

Spheric mirror projection for your own dome

Graphical interface and extensive keyboard control

Telescope control

Visualization:

Equatorial and azimuthal grids

Star twinkling

Shooting stars

Eclipse simulation

Skinnable landscapes, now with spheric panorama projection

Customizability:

Add your deep-sky objects, landscapes, constellation images, and scripts