Since Portal 2's sprawling, award-winning soundtrack was made available as a free download on the game's official website soon after its April 2011 release, a version that fans had to pay for had to be something special. The four-disc Collector's Edition of Songs to Test By definitely caters to the kind of Portal fan who loves to have souvenirs of the game -- official and unofficial Portal-related merchandise runs the gamut from mugs and T-shirts to socks and jewelry -- by including "Turret Lullaby," a mini-comic in which the game's fatally precise turret bots are tested for nursery duty; album artwork featuring Portal's protagonist Chell and her electronic frenemies GlaDOS and Wheatley; download keys for two other Valve games, Team Fortress 2 and DOTA 2; and perhaps most notably, the soundtrack for the first Portal game, which features not one but two versions of Jonathan Coulton's cult favorite song "Still Alive." That song and its Portal 2 sequel, GlaDOS' farewell "Want You Gone," are two of Songs to Test By's standouts, but they're far from the only ones. Appropriate to the game's many levels and layers, Valve composer Mike Morasky's music ranges from subtle audio backdrops for puzzle-solving ("15 Acres of Broken Glass") to tense action cues ("The Part Where He Kills You") to eerie mood pieces ("Robot Ghost Story") to tracks that hint at the game's surprisingly emotional back-story ("Ghost of Rattman," "Caroline Deleted"). Other tracks, such as "Machiavellian Bach," are noteworthy for how seamlessly they blend flowing classical and stiff electronic motifs, or for how well they express the game's retro-futuristic vibe, like the sprightly "Halls of Science 4." As on "Still Alive" and "Want You Gone," former opera singer Ellen McLain's vocals grace some of the soundtrack's most evocative and emotive moments, whether as GlaDOS on the beautifully eerie "PotatOS' Lament" or as a chorus of turrets on "Turret Wife Serenade" and "Cara Mia Addio." Indeed, every aspect of both Portal games has so much personality that it's something of a shame that Songs to Test By doesn't include some of the more hilarious excerpts of the games' consistently witty dialogue as another bonus. Nevertheless, this set delivers as an extra-special, extra-geeky presentation of one of the game's most vital elements.