iOS: Apple's Genius does a good job at suggesting songs that you might like based on your ratings, but Groove is a music player that does the work for you. All you have to do is listen to your music, and the app takes over from there. It'll build custom playlists based on what you like, act like a personal DJ, automatically tag your tunes, download album art for you, and more.
One of the best features of Groove is that the app can automatically make you a playlist based on one of the songs in your library with a single tap. That may not sound special, but the key here is that the playlist is actually good. When I tested the app, I was surprised at how well Groove was able to match up songs that I had listened to or lost in the depths of my music library. The app can also build you a "surprise me" playlist where it'll pull together different tracks every time you build the playlist.
If your library is a mess, Groove can also automatically tag and organize your music so you never have trouble finding those songs in the future. The app uses data from Last.fm to download album art, artist bios, and tags, and will scrobble back to Last.fm as you listen. Groove is a good looking player, and if you're looking for a smarter jukebox app for your iPhone or iPad, it's worth a look. It's available now in the iTunes App Store for $1.99.
Groove 2 | iTunes App Store via Groove Music Player