Gizmodo posted a list of the apps used in the video:
"Carol of the Bells" SoundGrid NLogFree Melody Bell Guitarist (Nylon String) iGog (MoreVox Acoustic 1 sound) Bassist Guitarist (Electric)
"Rockin' Around the Christmas Tree" iGog Bassist Guitarist Pocket Organ Saxophone Musicofx Percussions (Tambourine)
"Feliz Navidad" Percussions iGog (Rock Kit 1 sound) Bassist Guitarist Bebot Pianist T-Pain
Can it be long before iOS devices are used for Christmas caroling? Oh, wait.