Isolator (hide Mac OS X desktop icons)

Isolator is a small menu bar application that helps you concentrate. When you're working on a document, and don't want to be distracted, turn on Isolator. It will cover up your desktop and all the icons on it, as well as the windows of all your other applications, so you can concentrate on the task in hand.

Isolator is designed to stay out of your way, and so it isn't loaded with flashy features. However, it is customizable so you can get it to do exactly what you want.