The iPhone's default Music app already has lyrics support built in, but you have to fetch the lyrics yourself; songs sold on the iTunes Store still don't include lyrics after all these years. Shazam, makers of the well-known app that will identify songs playing in your vicinity after sampling a few seconds of audio, has released a free music player app called Shazam Player for the iPhone and iPod touch that addresses this gap in a big way.
When first launched, Shazam Player will scan your entire music library to find songs compatible with its LyricPlay feature. Depending on how big your library is, this could take awhile; my iPhone 4S has close to 5000 songs on it (though for some reason Shazam Player only scanned 1600 or so), and it took about 5 minutes for the app to finish scanning my library for the first time.
Shazam's lyrics database doesn't appear to be especially comprehensive yet, or at least not for the kind of music I tend to listen to. Out of 4936 tracks, Shazam Player found only 374 compatible with LyricPlay.
For songs that support it, LyricPlay is pretty cool and mildly reminiscent of the kinetic typography music videos I've seen on YouTube. Lyrics scroll or fly onto the screen perfectly in time with the music. There are a variety of fonts and animation styles in LyricPlay, with some of them easier to read than others -- one particular style displays the currently playing snippet of lyrics at the bottom of the screen with all other lyrics jammed above it, which isn't particularly readable. Fortunately Shazam Player also includes Lyrics sheet, a more standard text-based view of lyrics for when LyricPlay wears out its welcome.Shazam Player includes some other welcome features like links to YouTube videos of the currently selected song and tour dates for the band. Since it's integrated with your music library, you have full access to every song, artist, and playlist on your device. However, the app doesn't appear to support nested playlist folders, instead presenting all playlists in the same view; if you're like me and have many playlists, Shazam Player's playlist view could seem a bit disorganized.
Shazam Player has a nice UI, and the price is right -- since it's free, there's no reason not to check it out. But while LyricPlay and the other features are novel features, I'll most likely stick with the standard Music app on my iPhone; Shazam Player seems to lag and choke a bit on a music library as large as mine.
That having been said, I strongly hope that Shazam is working on a version of Shazam Player for the iPad. While I don't feel that Shazam Player is going to replace the Music app on my iPhone in daily usage, the Music app on the iPad is so terrible that something like Shazam Player would be a most welcome alternative.