"Joy to the World!" by The Butties
You don't think that the world needs a Beatles cover band? And that, if it should exist, it shouldn't be fronted by the guy who plays Ted on Scrubs? Just listen to this song and try not to smile.
"God Rest Ye Merry Gentlemen/ We Three Kings" by The Barenaked Ladies with Sarah McLachlan
I love this stripped-down mash-up of these two songs. Not only do they go together surprisingly well, but the bluesy tone taken by the singers and the musicians give it a quality that isn't often found in really good Christmas songs (usually, when Christmas songs take this route, it sounds like they are trying too hard to be cool).
"Another Christmas Song" by Stephen Colbert
No matter how many times I see the video or hear this song, I always crack up a little. Besides Colbert's manic joy, the lyrics are a delightfully metacognitive take on the commercial viability of Christmas songs. Colbert pleads with us to buy his song while including every Christmas cliche possible (sample lyric: "Chestnuts glisten on a silent night/ Sleigh bells kissin' by candlelight/ The tree is frozen, the winter's bright/ Who'd have thought the wise man looked so white?"). Genius.