Valve has launched the Steam Music Player, which lets you listen to your music collection while you play games on the digital platform.
With it you can point Steam to the MP3s on your computer, then browse your collection of albums and artists. You can listen to albums, queue up mixes and create playlists.
Steam Soundtracks downloadable content appears in your Steam Music library when the associated base game is installed.
To coincide with the launch of Steam Music, Valve has made selected Half-Life and Portal soundtracks DLC available for free to those who own the related base games and documentary. And while we're at it, here's Steam's list of soundtrack DLC.