Snowplane is an animated desktop wallpaper for the Mac. It is an absolutely free app (in fact it is open-source) that shows how snow falls in the background of your Mac. The application doesn't have a graphical user interface, and all you can change can be changed from the menu bar icon. That's where you close the application as well.
By default, the amount of snow that falls is set to medium, but you can change that from the menu bar icon. The snow falls very slowly until it reaches the bottom of the screen. This snow is basically white dots of different sizes. When this app is active, your desktop wallpaper is darkened in the background, but it still is visible. The snow doesn't go on top of applications or desktop icons. I noticed a few times that the effect flickered at times, especially when you are switching applications. Of course this isn't a big deal.
Snowplane worked fine for me and I have a dual-screen configuration. I could see snow on both my screens.
All in all, if you must have an animated wallpaper, I guess this one is as good as any other. It is free, silent and it runs well most of the time.