As it's name indicates, Stellarium is a spectacular planetarium, very complete and above all, free. The program incorporates more than 120.000 stars, planets, constellations, nebulae, various effects, etc.
Stellarium also has a complete navigation system and a complete customizable visualization of the Earth. Furthermore, it combines a powerful database of the sky with photo-realistic visualization options.
In only 40MB you will have a complete compendium of the sky and a different way of gazing at the stars.
In addition, thanks to this portable version, you can enjoy the original program in any device with no need of having it installed.