Stellarium is an open source sky simulator that allows you to view the constellations as they appear from any place on Earth, at any time you choose.

It's a beautiful piece of software, with fantastic sunsets and night skies (as you can see from the screenshots. I'm still exploring it -- I'm still not sure how to get close-ups of planets and nebula -- but so far its definitely appealing to the inner geek who always loved looking at the star charts in Astronomy and Odyssey.

The software -- which is still in beta -- runs on Windows XP, Linux and Mac OS X.