TheAppleBlog

That mosaic screen saver of your iPhoto pictures may look groovy, but it’s really not doing anything to help your work flow. How about a screen saver that displays all your iCal appointments instead?
PocketMac, the geniuses behind the app that lets you sync your BlackBerry and Mac, has released iCalScreenSaver. This fully configurable app lets you [...]

Apple screen savers are pretty much just Quartz Compositions (graphics created via Quartz Composer, a node based visual programming language), and, while there are some nice ones that come pre-installed, they get old rather quickly. You could opt to dedicate those idle CPU cycles to an altruistic effort (or just put your system to sleep [...]