KeyStrokes: Invert Your Screen

CONTROL + OPTION + COMMAND + 8

Hitting these keys will invert the colors of your screen. White becomes black, and vice versa. And the same goes for every other color on your screen as well — it looks rather cool. Pressing the keys again will return the colors to normal.

There are really two uses for this one:

1. Accessibility: Some people may have a hard time reading text on a regular white background, or perhaps have trouble with the opposite. Either way, all you have to do is hit this key combo. From what I hear, this really helps a lot of people. In fact, it must help people so much so that Apple now includes this feature in the Accessibility options in iOS 5. Just head to Settings —> General —> Accessibility —> White on Black.
2. As a joke: Want to welcome a new Mac user? Just hit this key combo when they're not looking. They will have no idea how to flip it back, and will begin to nuts thinking their brand new computer just broke. Sit back, and watch the hilarity ensue.