Black and White.
Can you get better than black and white? Not in my opinion. That's why I was pleased to discover Nocturne, a small app from the people at Blacktree. They developed Quicksilver, another app that I did not play with much, as at a glance it seemed to be similar to the upgraded Finder, offering the features that it now has but didn't in OS X 10.4. I am sure it has many more advanced features that I did not have time to discover, but back to my previous point.
Nocturne is an app that allows you to make your screen monochrome, in shades of black and white. It also offers a feature to invert the colours, but this can also be achieved with OS X by using the Control-Option-Command-8 trigger. It's a neat little app for when you're hunched over the screen late at night, and the colour of the screen just grits with the atmosphere.
Though it does not serve much of a practical purpose, it is a neat little app that now has a nice little place in my taskbar.