If you like the iTunes Artwork screen saver on your Mac, you will probably like Wall Of Sound (Free) by NuageProduction.
The app does what the title suggests: it takes the album artwork from the music in your iTunes library and creates a "wall" on your iPhone or iPad screen. You can limit said wall to a specific genre, or let it all in.
You can drag the wall in any direction, even diagonally, and more albums will appear. You can also pinch to resize the albums. Once you find something you want to listen to, simply tap the album. It will bring up a detailed view and start playing the first song.
From the album view, you can also let people know what you're listening to by sharing via email, SMS, Facebook, or Twitter. If you want to keep browsing, just swipe up on the album and it'll disappear, leaving only playback controls.
In addition to your own iTunes library, you can opt to mix in albums by top artists from the iTunes Store. This option allows you to find new music, preview it, and even buy it directly from within the app.
It's a nice feature, but not quite the music discovery experience the developers would have you believe it is. Yes, you can find new music, but it's not nearly like the experience you'd get from something like Pandora.
Nevertheless, Wall of Sound is a well-designed interface for listening to music you already have. If you've been looking for an alternative to the standard music player, this could be it. For free, you might as well give it a shot.