Used to use a screensaver collection called AfterDark (back in the days of Windows 3.1), which had a fair number of nice fluid moving things like that. This was of course when I was using a portable machine with a plasma display - so not having static images on the screen any longer than possible was actually an issue for me!
These days I've just got the default Windows XP one on. Had a textured flag of my desktop wallpaper for a while - but that used rather a lot of CPU time (rubbish graphics card), and would cause the CPU fan to speed up and make a noise - which annoyed me. Monitor's also set to go into standby after 15 minutes. The simple and resource friendly approach.
The Red Hat (8.2) distribution of Linux I was running on my old PC for a long while had a nice selection of screensavers, the one thing I noticed was that they were all *incredibly* fluid in the motion compared to anything I've seen in Windows. Plus it had those fractal creation things, which I could quite easily get utterly hypnotised by.