Valve decided to send Steam Music into its public release yesterday. To celebrate the launch of the new service, they're offering a bunch of free soundtracks and a 75% off sale on a number of their titles.
If you're wondering how to get your hands on Steam Music, just update the Steam client and look under the library tab or open Big Picture mode. Currently it does seem a bit difficult to add new music to the library, but Valve has promised to continue accepting feedback from the community and improving their newest feature.
As for the sale, there are some great Valve titles for dirt cheap, so you all don't have an excuse to never have played Portal. Also, soundtracks are freely available from: Free to Play, Half-Life, Half-Life 2, Half-Life 2: Episode One, Half-Life 2: Episode Two, Portal, and Portal 2. You can also find a ton of other great soundtracks at a low price on Steam. The sale ends on October 1st.
If you're looking for a recommendation, I'm really loving the OST for Crypt of the Necrodancer.