Here are a couple of my favorites:
John Denver and the Muppets do the 12 Days of Christmas like nobody's business. Even though the video is blurry, it's still worth watching. In college, a girl who lived in my hallway had a taped copy of John Denver and the Muppets Christmas special that she had made a tape recorder next to a record player that was playing the album. While it was recording, she left the room and the record started skipping, right in the middle of Miss Piggy singing "five gold rings." She walked back into the room and fixed the problem, but the taped copy had Miss Piggy singing "GO-old" fifteen times. It was spectacular.
Fairytale of New York by the Pogues with Kristy McColl. If I ever meet a guy who will get drunk with me and sing a karaoke duet of this song on Christmas eve, well, I'd probably marry him. Even if he had teeth like Shane McGowan.