From Matthieu Riolo:
Screenwatcher is a small cocoa application that displays a movie or a screensaver as the desktop background. The application contains a small library interface for the movies. When the application starts it reads all screensavers that are installed and displays them in the corresponding list (this way you can use any screensaver and you can install them absolutely normal by a double-click). It is opensource and can be modified by anybody. Because Screenwatcher is based on the QTKit (well known as Quicktime) you can extend the movie player with any Quicktime component. As example you could install the component Perian if you want to play some .flv movies.