Christmas Canon by the Trans-Siberian Orchestra. Love it.
I admit to not knowing much about the group or how I feel about a lot of their rock-themed Christmas music...but this piece has just stolen my heart and is now my 2009 Christmas favorite. I can't help it. If anyone has a favorite Christmas piece that they think can rival this one, please share it...because I LOVE CHRISTMAS MUSIC (and unashamedly listen to it for a week in March, July, September, then the whole months of November and December). :D
The video is available on YouTube, but the embedding feature has been disabled, so here is the URL: http://www.youtube.com/watch?v=4cP26ndrmtg
...if that doesn't work, then just search for it on YouTube...but don't watch the rock version.