I think this might have gotten a little funky at the end. I kinda looped this slipshoddedly, so after about an hour, it might have 3 minutes of silence, then play the song, a few times. This and "No Cake For You" are my favorites from the Portal OST. It's just so wonderfully ambient that one just has to loop it nonstop!
I'm a sucker for pure ambient music, like this and the Antichamber soundtrack.