After seven months of beta testing, Steam's integrated music player has finally been integrated into the official client. The addition is part of the Steam update released yesterday; it includes basic playback, playlist, and library functionality that can be accessed through the desktop interface, Big Picture mode, and the in-game Steam overlay.
Folks with the update installed should see a music section in the Steam library. DLC soundtracks for installed games will show up there automatically, according to Valve, and users can add their own tunes via the settings menu. Networked sources are only supported with mapped drive letters, though. The music player also seems to be limited to MP3s.
As one might expect, the Steam Music discussion forum is already filled with requests for broader format support and streaming integration, among other features. And Valve is apparently listening. In the post announcing the initial beta, company staffer Christen Coomer said that "feedback will inform what becomes of Steam Music" and "help guide where we take things next."
Given Valve's history of soliciting—and actually listening to—user input, Steam could well become good all-around music player. I can already think of a few games whose soundtracks have worn thin, so I'll definitely be using it.