This is a port to Windows of Flurry, a beautiful screen saver for Mac OS X written by Calum Robinson. To install it, extract the .scr file from the zipped archive, right click it, and choose install. You can then configure it from the display properties screen.
Five different flurries are available: classic, RGB, fire, water, and psychedelic. It also supports multiple monitor configirations and can display one massive flurry across both or a different one on each monitor. The visuals are fantastic, especially considering the download is only 54kb.
The .scr file scanned clean with ESET Smart Security v4 beta and on novirusthanks.com.
More info: Flurry screen saver