Pinna is a minimalistic but robust audio player for the Mac. It was designed to let you listen to all your music without getting in the way, something that iTunes tends to do a bit. The first thing you will notice about Pinna is that its user interface is rather small. This is by design, to make browsing your music and listening to it as easy as possible. All your iTunes songs and playlists can be accessed with Pinna, which makes the transition to this application quite easy. However, you can't add new songs to Pinna, you will still have to do that in iTunes. You can double click songs to start playing them and add them to your queue. This comes in handy when you want to listen to a few songs but don't want to bother creating a playlist. If you don't want to do this, you can simply click the shuffle button and Pinna will select music for you randomly.
By default, when Pinna is on the background, it will switch to album art mode, displaying the cover of the album of the song that is playing. It has other nice features, for example, it will pause playback if you remove your headphones. It can also use your keyboard function keys to control playback if you enable this in the Preferences.
In short, Pinna is a nice alternative to using iTunes to listen to music. It is relatively faster and easier to navigate than Apple's solution, but it still requires it to add music to your libraries.