Znow

Znow is a fun application that you can use to create different special effects on your screen like snow, rain, and wind.

Besides these rather common effects, you can also try some more unique ones: a beer effect, which makes your screen look like a pint of ale; a Valentine's Day effect, which fills your screen with little hearts; and a star effect, which decorates it with tiny stars.

From the application's main window, you can also set certain parameters, such as the opacity or direction of the elements you've added – although the most interesting option might be the ability to change it into a screensaver.

Znow is a pretty showy application that may or may not be useful. It's an absolutely gorgeous screensaver, though, if that's what you're looking for.