It's just a simple and beautiful binary clock made with Quartz Composer.
To use it as a screen saver on Mac OS X, just put the file BinaryClock.qtz in /System/Library/Screen Savers/ or in ~/Library/Screen Savers/.
After that, you can now select it in your System preferences in the pane Desktop and Screen Savers.
You can also double-click on the BinaryClock.saver package in the Finder to install a compiled version of the screensaver, ie with a more beautiful configuration panel than the .qtz version.
Source code of the compiled version is available in the Xcode directory.