I personaly love music and almost listening to music at anytime. So, my favorite mobile app is Spotify.
Spotify is not necessarily a mobile app, we can use it too in laptop or computer. But, i find it really handy as a music application especially on your phone.
I used to use itunes on my phone and laptop, but then it becomes very annoying to keep downloading new songs. While spotify allows you to find your latest music from other spotify users without downloading them. You can make your own playlist and make them private or public so that your friend can access it too. It is kind a half social media, half music player application. When connected to facebook, you can find your friend and start following them listen to their playlist, even see what they are listening to at the moment from laptop app. Listening to music according to your mood or genre would be so much easier.