VidRhythm is a silly little iOS thing that's a lot of fun to play with. I created the above corgi music video with it. I've already recommended it to you, but I'm giving you another push now as it's free today.
Harmonix's video-making/song remixing app has 12 new tracks as of today, including holiday songs like "Jingle Bells" and "Deck the Halls." There's also built-in pitch correction for your crappy voice. Video functionality has also seen an upgrade; now you can export 720p video to YouTube.
Again, VidRhythm is free for iPhone and iPad today. Get it now.