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 it’s definitely appealing to the inner geek who always loved looking at the star charts in Astronomy and Odyssey.
The software runs on Windows XP, Linux and Mac OS X.