Invert the Screen on Mac OS XFrom http://www.patriceschneider.com/apple-osx/blog/?p=8
Another pretty cool one is to hit the keyboard combination “Control ALT Apple 8″ which will invert the colours on your screen. (Hitting the same combination will change it back to the standard setting.) This can at times also give you an extra 30 minutes or more. It can under certain bright lights also make it easier to read text, btw. Give it a try!