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.

Features:

sky:

default catalogue of over 600,000 stars

extra catalogues with more than 210 million stars

asterisms and illustrations of the constellations

images of nebulae (full Messier catalogue)

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

visualisation:

equatorial and azimuthal grids

star twinkling

shooting stars

eclipse simulation

skinnable landscapes, now with spheric panorama projection

customisability:

add your own deep sky objects, landscapes, constellation images, scripts...