In the spirit of the 4th of July, here is a great fireworks screensaver called Skyrocket. Mac version. Windows version. You can see a video of the screensaver here.

The fireworks are rendered in 3D with all sorts of customization. Macintosh users: Be sure to turn on the sound (and tweak other features) by clicking on the Options button under the preview screen in the Desktop & Screen Saver system preference.