Volumio (http://volumio.org/) is a free, open source Linux distribution based on Raspbian, designed to turn a Raspberry Pi into an audiophile music player. Volumio is designed to be easy to use. To quote the Volumio website, Volumio can be defined as follows:
"Volumio is designed to be simple. To sound amazing. To be what your music deserves."
After installing Volumio onto your Raspberry Pi, you can control the music that you are playing using any smartphone or tablet with Wi-Fi.
The interesting thing about Volumio is that, as it has been designed for use by audiophiles (people who enjoy high-quality audio), it is designed to be able to use external digital-to-analog converters (D/A) to increase the quality of the sound being played. These D/A converters plug into your Raspberry Pi and work as a very high-quality sound card. The following image shows the Volumio web interface:
