Dream Aquarium Screensaver
Full user review
"Not bad but not worth the price"
Very pretty, highly configurable, very realistic
Nothing works if you don't pay the full price (i.e. don't believe it's an "evaluation" copy - you can't get it out of demo mode), and the full price is too much for a screen saver.
When I am at my desk I'm using my computer, so I hardly ever see the screen saver. I'm not going to pay $13 or however much they want for this, so I can entertain my co-workers.