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.
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
a powerful zoom
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
equatorial and azimuthal grids
skinnable landscapes, now with spheric panorama projection
add your own deep sky objects, landscapes, constellation images, scripts...