Really Slick Screensavers - http://reallyslick.demonews.com/
This guy did some amazing opengl screensavers. Still on his site, including the source, but I don't think he's doing any new ones. Has about 8 or so now - all *very* good. All adjustable so you can create almost infinite variations. Has a great fireworks one.
Also, Serenescreen (the fish tank that has the 3 fish demo with XP).