SereneScreen Marine Aquarium is a realistic, high-definition screensaver that converts your computer screen into a vibrant, colorful saltwater aquarium. You can choose from 27 different exotic fish species--from the Addis Butterflyfish to the Yellow Tang--and SereneScreen can display up to 30 fish at a time, with an easy interface for creating and customizing your own sets of fish (including an option for random species). You also have settings for autopanning, framerate, iTunes playlists, and a bubble column (from a virtual aerator, with optional bubble sound).
SereneScreen has an optional "crystal" paperweight in the tank, too, which you can use to display a calendar, clock (digital or analog), or a custom image (the app suggests a logo, but you can use any image). SereneScreen's superwide virtual tank also now supports up to three monitors simultaneously.
Screensavers are of course unnecessary with modern displays (since screen burn is no longer a problem) and the Mac OS comes with multiple free screensavers built in. That said, if you're looking for an entertaining and impressively eye-catching way to occupy your idle monitors, SereneScreen is a great choice.