If you're planning to go out singing carols this Christmas then maybe you should install xmasMusic on your phone first.
This free app offers you the lyrics to a wide range of festive songs. You simply choose the letter of the alphabet of the song title then select it from the list and the words to the tune will be displayed on screen. xmasMusic covers everything from All I want for Christmas is my two front teeth to Silent night. The songs are generally either hymns or traditional, so don't expect any modern stuff from the likes of Slade or Cliff Richard.
Unfortunately, xmasMusic actually offer any music to accompany the lyrics, only the words to the songs. In fact, it doesn't even do this particularly well, given that a lot of the lyrics seem to be left off the end of many of the songs.
Another improvement I think that could've been made to the xmasMusic would be to inject some Christmasness into its dull and boring interface (which, incidentally doesn't have any options or settings).
xmasMusic is a nice idea for an application, it's just a shame it's been executed so poorly.