All three verses of O Holy Night.  (I prefer the meaning of the English lyrics, but I love the sound of the original French version --Cantique de Nöel--, which is from about the 15th C., I think.)

Riu, Riu, Chiu

Virgin Mary  (as sung by Carolyn Hester)

There's A Song In The Air  (a hymn that was always in our Baptist Hymnal when I was growing up).  The first verse goes:

There's a song in the air, there's a star in the sky,
There's a mother's deep prayer and a baby's low cry,
And the star rains its fire while the beautiful sing,
And the manger of Bethlehem cradles a king.

It Came Upon The Midnight Clear   -- especially the verse that goes:

Still thro' the cloven skies they come, with peaceful wings unfurled,
And still their heav'nly music floats o'er all the weary world.
Above its sad and lonely plain they bend on hov'ring wing,
And ever o'er its Babel sounds the blessed angels sing!

Es Ist Ein' Ros' Entsprungen   (Lo, How A Rose E'er Blooming)  -- this is so pretty auf Deutsch!

Liland, Thanks for posting Longfellow's oft-omitted verses. There is a fuller discussion of his poem, and how it came to be the carol we know, in another thread here at Mudcat (from sometime this fall.)