When in a dark environment where your screen is too bright, or for users that do a lot of photo-editing, it can be useful to invert your screen’s colors. This can easily be done by pressing the following key-combo:
command + control + alt/option + 8
Press the keys again to revert back to normal.
For those who have updated to Mountain Lion (Mac OS 10.8.x), you will find that this shortcut no longer exists. Simply go to System Preferences > Keyboard > Accessibility and tick Invert colors to bring back the option.