Back in the day when 3d accelerators were brand new (remember the Voodoo 1?), VRMan made some incredibly innovative screensavers. I remember watching the Christmas screensaver for hours on my brand new Voodo 2 with Pentium-90.
http://www.vrman3d.com
The site went offline for a while, and these screensavers were hard to find. But the site is back, and he has some brand new screensavers, some with PhysX support.
The screensavers initially have a problem running in Windows Vista, because Vista no longer has the d3drm.dll (which you can download at
http://www.driverskit.com/dll/d3drm.dll/494.html). Once I found this DLL and copied it to the c:\windows\system32 directory, the screensavers worked just fine!
These screensavers once pushed the limit of the 3dfx Voodoo 2: Taste my GeForce 8800!