I felt it was appropriate to mention that the entire Portal 2 soundtrack is now finally available in its entirety from the Portal 2 website. You can download it here.
Past volumes have released tracks in the game's chronological order. Only tracks from the beginning were available in the first volume, then in the second volume tracks from the middle of the game were maid available. Volume 3 contains all the edited score from the end of the game, including the end boss theme, the music for SPAAAAAAACE!!!! and the ever important "Want You Gone" and "Cara Mia Addio" tracks. If you didn't almost burst into tears when you heard the latter track in the game, you probably don't have a soul.
Volume 1 was probably my favorite out of the three. Though the third volume has obvious things going for it, as far as score goes I liked 1 better. 2 is mostly boring, but I still think the entire Portal 2 score is impressive for a video game. I haven't listened to all of 3 yet, but I'm sure it is quite nice. You can use the above link to download all the score pieces.