Someone already mentioned Barenaked Ladies cover of God Rest Ye Merry Gentlemen-it's one of the best updates of a really old song ever.

Also, if you're not familiar with them, Trans Siberian Orchestra rocks hardcore and outloud. I'm particularly fond of Christmas Eve (Sarajevo 12/24), but the entire album is rock remixed Christmas classics. I swear, it makes the Nutcracker Suite worth listening to. If you can ever get a chance to see it, the music video to Christmas Eve (Sarajevo 12/24) is one of the coolest video presentations ever.