Of all the currently released Twitter screensavers Twitsaver is definitely one of the more interesting ones. Unlike Twitter Screensavers like Flitter or TwittEarth that concentrate on the locations and messages that are send by Twitter users Twitsaver does concentrate on images that get referenced by Twitter users.
The homepage does not contain information about the criteria [...]
TwitSaver Twitter Screensaver
Flitter a Flickr Twitter realtime screensaver
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.
