Valve's in the process of testing something called Steam Music. Before you freak out and worry that Valve's getting into the music business, relax: this isn't a Spotify competitor. It's just a way to integrate your existing music collection with Steam.
It's a pretty simple idea. You tell Steam which directory your music is in, and Steam can then play it all while you're gaming, including album art.
This might sound a bit naff for most games, but for more ponderous experiences - I'm looking at you, Civ & Total War - the ability to play my own music instead of listening to the same score for the 1,455,346st time, will be blessed relief
Steam Music [Valve]