xSnow is a GPLed Desktop Toy by Rick Jansen. It is also the greatest program ever written, the Killer App of Linux, and the best reason to hack perl inside of Xwindows.
Essentially, it makes your desktop snow; snow that falls down from the top of the screen, swirls around, and sticks to whatever it hits: the tops of windows, the bottom of the screen. But it goes beyond that. xSnow includes the sporadic appearance of Santa and his sleigh, with or without Rudolph!
xSnow has been magically pre-installed on every Red Hat box that I've used. I'm unsure of other Distributions. In any case, binaries are availabe at http://linux.tucows.com/preview/10297.html.
xSnow can be run from a console window inside of X-Windows using the command "xSnow". There's lots of options, so I suggest you check out "man xsnow". One particular combination that I've found amusing is "
xsnow -snowflakes 1000 -notrees -xspeed 6 -yspeed 12", which removes the trees, and increases the amount and speed of the snowflakes dramatically.
Getting it to run with GNOME
Sadly, some versions of xSnow don't work properly immediately on when running the GNOME window manager. This is because Nautilus, by default, draws icons over top of the root desktop, blocking out the layer that xSnow is shown on. This can be solved easily, but with the trade-off that Nautilus will no longer draw pretty desktop icons. Open Nautilus, go to preferences (make sure you're on "intermediate" or "advanced" options mode), and unchecking the "use Nautilus to draw background" item.
This problem has appeared in KDE, or with other programs that draw over the root desktop. Apparently, the version of xSnow that ships with new versions of RedHat solves this.