Used to be agencies did amazingly cool holiday cards because it was the easiest way to get something into the One Show without a client futzing with the concept.
Now that self-promo pieces are mostly abolished, agencies do it because they're still fun to work on.
Here are a few from this season. Any others you know of? Post them in the comments.
http://evbholidayfax.com/
http://holidaycard.clickhere.com/
http://www.youtube.com/watch?v=GkHNNPM7pJA
http://digitalfruitcake.com/#/home (This was from last year, but it still cracks me up.)