Always been fond of "Twelve Days" spoofs, such as:
(nice animation added to this classic).
and, from John Valby, the NSFW version:
and the Jeff Foxworthy version:
"Somebody done been to the Wal-Mart!" LOL
There's lots more versions. Can anybody find some others?
Billy Crystal's version of The Christmas Song is pretty good too.
Not a version of a regular Christmas song, but still weird, is Ray Stevens' Santa Claus is Watching You.