Drive Software's Butterfly On Desktop places realistic-looking butterflies on your Windows desktop. It's not a screensaver, though it can serve in place of one if you open it when you leave your PC. Like real butterflies, Butterfly On Desktop exists purely for your pleasure. It's free and runs in most versions of Windows.
We installed and opened Butterfly On Desktop, which opened with a brace of monarch butterflies flitting around our screen. Their motion is adequately realistic, and the effect is nice. A system tray icon let us add butterflies one by one or in groups of five, close all butterflies, and open the Settings. By default, Butterfly On Desktop opens with Windows and stays on top; we could uncheck those options, as well as set the default number of butterflies from one to 99. In theory, that is: We tried to literally cover our desktop in butterflies, but the sims proved too much for our two CPU cores and 8GB of RAM, and the butterflies became sluggish and jerky in motion. It seems that Butterfly On Desktop couldn't load more than 20 or so butterflies at once, and when we clicked "Close All Butterflies" on the program's menu, a new set of butterflies would open as soon as the previous one closed. We had to Exit out to stop the madness! Reducing the count set things straight.
Of course, programs like Butterfly On Desktop are strictly a matter of taste. It's a pretty effect, especially from a distance. You could uncheck the option to start with Windows and launch Butterfly On Desktop when you leave your PC to use the program as a screensaver, though proper screensaver options would be welcome. It'd be nice to be able to change the color (or species) of the butterflies, too. But Butterfly On Desktop is what it is: not necessary, but nice.