If you are looking for a rather unique screensaver for Windows 2000, XP or Windows Vista you should take a look at Flitter which is a mashup from Flickr and Twitter at the moment. What it does is that it is retrieving realtime data from both Flickr and Twitter to display those when the screensaver is active.
You obviously need a computer that is connected to the Internet to use this screensaver but it is very well worth it. The default setting for the screensaver pulls data from the public timelines of both Flickr and Twitter but you have the possibility to custom this further.