Celestia

[Cortona]

I used to love this when I had a PC, it was fantastic. When I switched I didn't think it was available for OS X but I see now that it is. (I have a similar but far inferior free program called Starry Night Backyard.)

I'll have to download Celestia and see if I can get it running on my ancient and hopelessly obsolete Mac.

[Bloodwych]

As Shoonay mentioned, if you are just after something to allow you to identify all those wondrous stars and planets on a clear night, Stellarium is what you're after.

Very lightweight and simple to use, you can literally load it up and be identifying the night sky like a pro in seconds:

http://www.stellarium.org/

Jupiter is very bright in the south at the moment. I can see it out of my window.

Celestia is good, but more than I require most of the time.