As the name suggests, Equalizer+ allows you to customize your music to your mood using five different frequency filters and different sound presets. We particularly like the bass booster and the visualizer functions.
Available on iOS and Android with in-app purchases 
If your music library is getting out of hand, the newly-launched Groove might just be the answer. With its nice, easy-to-use interface, the app arranges personalized playlists based on your listening habits, including old favorites that sometimes fall out of rotation.
Available on iOS and Android 
Edjing offers a database of over 50 million tracks via Soundcloud and Deezer, which you can download and add to your own mixes, making use of the realistic DJ effects (turntables and all). You can then share these over Facebook, Twitter and Google+. Available on iOS and Android with in-app purchases