One of my favourite songs at this time of year is O Come O Come Emmanuel. It may not be the most popular of carols (after all, it's really and advent carol rather than one for Christmas), but it is one which is both musically beautiful (at least in my rather subjective opinion) and, even more importantly, theologically rich. Over at the Gospel Coalition, J.V. Fesko (academic dean of Westminster Seminary California and author of Justification, one of this years best theological books) explains the theology behind the hymn. Have a read, it might help you see O Come O Come Emmanuel in a whole new light.
Whilst on the topic of seasonal songs, Justin Taylor has been highlighting a few good Christmas albums recently and giving free downloads of some of the songs. The albums include Advent Songs from Sojourn Music, Silent Night from Red Mountain Music (which includes a great contemporary recording of Let All Mortal Flesh Keep Silence, another of my favourite advent hymns), Savior from Sovereign Grace music, and Salvation is Created from Bifrost Arts.