SETI@Home was fun, but it was kind of a trash screen saver because of how many fixed elements it had. Definitely ended up burning in a monitor with the yellow afterimage of its purple section divider bars.
Later versions tried to counteract that by making it more a moving 3d thing of the original, LCARS-inspired interface. Incidentally that also was when I got sick of it.
And then later on Folding@Home was an interesting screensaver.