Valve is making the soundtrack from Portal 2, entitled "Songs To Test By", available to download free from the game's web site. The site also offers six ringtones to download.
The complete soundtrack will be released in three volumes; Valve has released one volume, comprising 22 tracks, so far.
A blog post on the game's website reads: "Whether you're a mega-science corporation with decades of test experience, or a young start-up liberating your first test subject from non-testing with a chloroform-soaked rag, Aperture guarantees results. Wake up your test subject, open the chamber door, and press play--let Aperture's patent-pending rhythmic compliance enhancers do the rest."
Portal 2 features music by Jonathon Coulton (composer of "Still Alive" from the original Portal), Mike Morasky and indie rock band The National.