As well as "Saving your screen" some screensavers do other jobs as well.
Searching for interesting radio signals, perhaps from alien intelligence?
http://setiathome.ssl.berkeley.edu/Entering code breaking competitions, and mathematical experiments
Couterpane's reaking weak 40bit email encryption
http://www.counterpane.com/
United Devices grid computing projects have a screensaver that looks for chemical matches for the smallpox virus
http://www.grid.org/projects/smallpox/
