These free screensavers require the program from Flashsavers.com (free, but you have to sign up) and the Flash Player from Macromedia (also free, and I believe it must be the player installed on your machine, not just the plugin), and it only works with current version of Windows. But keep an eye on Flashsavers.com, they might develop for other platforms eventually. If you have any questions about the Flashsavers program or the Flash Plugin/Player, go to the companies' sites.
Right click and save the target link to your hard drive, you can click the link to preview the file first (it will stretch to fit your screen). Save it to a place you can remember it, go to your display properties and hit the screen saver tab, select the Flashsaver (comes up as flshsvrs I believe) point it to the directory you saved your files in and hit "search". Pick one and set it as your screensaver. Note that this works for ALL .SWF files on your hard drive, so you can set your favorite saved Flash animation as your screen saver.