Also not strictly from Portal, but bothversions of 'Silence', from the main menu of the mod Portal: Prelude.
While playing through Portal, most people don't consciously realize there's music. It's so subtly woven into the ambient noise, it's barely even noticeable, except at a few very key moments (the radios and Still Alive, of course, but also the end of Chamber 19, getting the fully-functional portal gun, and so on).
Self Esteem Fund kicks in right about the time where the player begins to feel the isolation of Aperture Laboratories, having seen no one but Chell and heard no one but GLaDOS despite all the testing they have been doing. And this song perfectly augments that feeling... it is quiet and atmospheric, but solemn, reflective, and tugs at your heartstrings, making you feel truly alone in these bright walls of the testing facility.