If you own a Mac, you have got to try this out: a software called Nocturne by the same people who made QuickSilver. The software does a very simple thing – inverts your entire screen, giving the appearance of night vision. Looks great. Not only that, it is actually effective when using the computer in a dark room or at night! Not very good for surfing the web, though, since all photos are inverted as well.
(I had to take a snap since taking a screenshot only shows the original colours, I think Nocturne is only inverting the final display rather than the underlying buffer from which the screenshot is taken)
By the way, Mac OS X has a built in standard way of doing this – hit Ctrl+Option+Cmd+8. But Nocturne allows a number of options that make the effect more pleasing, like turning the screen monochrome, disabling shadows and applying color corrections.