I found these two great astronomy programs yesterday. Their both free and run on Windows, Macintosh OSX, and Linux. They use OpenGL so make sure you have up to date drivers installed or they will preform slow & laggy.
Celestia is a 3D representation in of the Milky Way and neighboring galaxys in real time. It has over 10GB of addons from models of the ships from Star Trek to tours of the solar system and various space missions. Theres alot of depth so make sure you go to Help>Controls and atleast learn the basics before you start exploring. I've only been useing this for a day so I've barely scratched the surface.
The second is Stellarium. It shows you a rendered night sky in real time. So if you want to know if a particular star, planet, nebula, or constellation is going to be visable tonight just type it in and press enter.