Really Slick Screensavers are the best 3D screensavers I've ever seen...and they ARE really slick! The author of the website and the screensavers write 3D OpenGL screensavers as a hobby, and you can see how he can be so passionate about it.
These savers splash your screen with brilliant colours, patterns and lights. As mesmorising as they are, some of them do get you a little dizzy (especially the one called "hyperspace", so do watch out!
They can be downloaded from the Really Slick Screensavers website under the 'Downloads' section in a zipped file for Windows or Mac. Enjoy!