Stellarium

Stellarium is an OpenGL application that acts as a real-time virtual planetarium, plotting realistic sky views similar to what you can see with your own eyes:

It is available for Linux, Windows and Mac OS X.

On a 20" iMac, edit Stellarium.app/Contents/Resources/config/config.ini to read:

[main]
version                        = 0.6.2

[video]
fullscreen                     = true
screen_w                       = 1680
screen_h                       = 1050
bbp_mode                       = 32
horizontal_offset              = 0
vertical_offset                = 0

...

...and don't try to change resolution via the GUI.