Stellarium

Whether you're a long-time, hard-core astronomy buff, or are just a part-time stargazer, you'll find something in this astronomy program for you.

It displays whatever celestial objects you want, including stars, planets, nebula, and constellations, and will display the sky in various grids and from different points of view.

Click any object to get details about it. Celestial objects move across the sky; you can have them move in excruciatingly slow real-time, or else speed them up. Overall, this is a beautiful and visually arresting program. It will draw pictures of the constellations, label them, and even let you change whether you want to see the night sky in darkness, or with some ambient light.

--Preston Gralla