You may already know that some of the screen savers in Mac OS X are Quartz Compositions. The RSS Visualizer, Arabesque and Spectrum were all made with Quartz Composer, which is included with the Developer Tools.

While those ones are automatically included as screen savers, there is actually a whole folder full of other compositions hidden in the system files. These files simply need to be dragged onto the Screen Saver section of System Preferences to install them.

To find these hidden screen savers, go to Macintosh HD, and navigate to /System/Library/Compositions. In here there are almost 90 Quartz Composition files. They aren't specifically made to be screen savers (it's actually a collection of iChat effects, slideshow transitions, image effects and iTunes visualizers) but some of them work just fine. When you find one you like, open up System Preferences and go to the Screen Saver section. Now just drag the file you have chosen onto the screen saver preview on the right. I have found that the following files work best:

Defocus.qtz
Fall.qtz
Floor.qtz
Grid.qtz
Image Hose.qtz
Kaleidescope.qtz
Moving Shapes.qtz
Stix.qtz
Travelator.qtz
Wall.qtz

Admittedly, Floor is pretty boring, and Stix is actually an iTunes visualizer, but they work none the less. Many of them allow you to change the options in from in System Preferences. For example, you have to choose an image for Kaleidoscope, or it will just display a blank screen.

There are even more compositions available in the Developer Tools. You can install the Developer Tools from the Mac OS X install disk. If you don't have the Developer Tools installed, it means you probably don't need them. It's overkill to install 2.5 GB of tools you will never use, just for a few screen savers. Instead, you can just install the examples. On the install disk, go to /Optional Installs/Xcode Tools/Packages and install DevExamples.pkg.

Take a look in /Developer/Examples/Quartz Composer/Compositions/Graphic Animations. Some of my personal favourites, "Scanner" and "Static" are in here. There are also two purpose built screen savers included with the Developer Tools, located in /Developer/Examples/Quartz Composer/Compositions/Screen Savers. "Retro" takes a picture of your current screen, gives it a sepia tone and bounces it up and down. "Security" takes pictures using your iSight and displays them in a grid. As with the other files, simply drag them to the screen saver preview in System Preferences to install them.