Today's tip for decorating your Mac is Snö. I first came across Snö about a year ago, and it's become a holiday staple for my iMac. Described as a "useless application" by its author (that's a bit harsh, I think), Snö produces a snowstorm on your active desktop (there's a screensaver included as well). Control how quickly the flakes spin as they fall, their opacity and shape, speed, size and so on. Note that the desktop app is pretty processor intensive, so you may want to stick with the screensaver. Snö is open source, free and requires Mac OS 10.1 or better.
Thanks to Appleology for reminding me of this!