Stellarium is a cross-platform planetarium app that has to be seen to be believed. Zoom through the night sky, check out constellations from anywhere on (or off) the planet, watch the rotation of the stars throughout virtual days, and more.
Stellarium is one of those open-source applications that makes you marvel at what open-source can accomplish. The interface is intuitive to use, the visuals are stunning, and the ease with which you can zoom through the stars (all 210 million of them cataloged in Stellarium) is amazing. Check out the video below to see it in action.