Stellarium is an open source desktop planetarium for Mac OS X, Linux/Unix, and Windows. It renders the skies in realtime using OpenGL, which means the skies will look exactly like what you see with your eyes, binoculars, or a small telescope.
As someone who is rediscovering astronomy after a lapse of about 30 years I must concur. This is a gorgeous bit of free software. I can conjure up a map of what’s in the sky and then walk outside to look for it with binoculars or telescope.
For added benefits bookmark NASA’s Satellite Tracking web page and you can find out what artificial objects are shooting through the sky as well.
Update, March 2009 — Fred emailed to say:
Stellarium has released a new version (0.10.2) and in addition to a much better program, there is now a plug-in (http://stellarium.org/wiki/index.php/Satellites_plugin) that lets you see the satellites that are in the sky (http://porpoisehead.net/misc/stellarium-satellites-plugin.exe). There is a Windows and Linux version.