It came upon a Midnight clear.
(Read all of the lyrics and see the date that they were written...) This year.
and of course
Silent Night.
Then, it isn't Christmas without,
Rudolph,
O little town of Bethelehem. Away in a Manger (Either melody), The Christmas Song (Torme), White Christmas .....Don't get me started.
Its awsome, I played it with my school a few weeks ago, sopraninos, recorders, altos, tenors, and a fife (I am the only fife that can really play that flute). It sounds really cool.
I also like In Dulci Jubilo which is quite simple but it is really cool too.
<Added>
Here is a video clip with a guy playing In Dulci Jubilo but in guitar, that doesnt matter, you just need to hear the song.
We use that melody for our "Alleluia" at Mass during Advent/Christmas season.
After a four measure piano intro, the flute (me) plays an eight measure "bell" part (the melody you commonly hear with this song). Then the singers start singing Alleluia (longer tone counter melody). The verses use the original melody. Of course the words are all liturgical in nature.
It's actually very beautiful, my description doesn't do it justice. We also have the whole orchestra version when we have strings etc.