If you are a Star Trek geek, then there is a screen saver called "System 47" by meWho that may very well be one of the coolest screen savers since Flying Toasters. It's available for both Mac & Windows, and is well worth checking out.
If, however, you are a Mac user with a multiple-monitor set-up, then "System 47" IS the coolest screen saver ever, and you should download it immediately (they brilliantly play the monitors separately, which was an unexpected bonus you don't get from most screen savers!).
Oh... did I forget to mention that it's freeware?
Now if you'll excuse me, I'm going to go back to pretending that I'm a Starfleet Captain...