Should’ve blogged this ages ago, but was using it a lot at Greenbelt and introduced quite a few people to it, so here it is –
if you need to either a) extend your battery life or b) see the screen in a really sunny place, you can invert the colour of the screen (like a negative of a photo) by holding down the control, alt and apple keys and pressing number 8. If the screen is predominantly white space, it saves lots on the battery, and you can then turn down the brightness of the screen (with the two buttons in the top left of the keyboard) and save lots of battery life.
I was shown this in LA in January by Janek, and used it a lot when trying to jack wifi from outside cafes in Hollywood. :o)by