Stellarium** is a free open source planetarium program 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.

If you're at all into telescopes or the night sky, this one's for you.

in version 0.8.0:

sky

over 120,000 stars from the Hipparcos catalogue with info
asterisms and illustrations of the constellations
images of nebulae
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

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...

Click the image to view a full size screenshot:

More great screenshots here.