If you simply can't be bothered unpacking that crusty box of Christmas decorations again this year, then at least deck out your desktop.
XMas Lights are a cheerful addition to your desktop to get you in the Christmas mood. The small bulbs hang just below your Menu Bar and light-up in red, yellow and blue. The lights flash at intervals specified manually in the settings, and you can also change the style of the lights to a smaller star-shaped design. If you're really feeling adventurous, you can design your own lights by designing your own images and then inserting them into '~/Library/Application Support/XMas Lights'. You can activate them simply by going into the preference panel.
If you're just a bit lazy when it comes to putting up real Christmas lights, XMas lights is a nice compromise.