Perhaps, like us, after the sheer joy of the Orange Box, you were left with a twinge of disappointment at the lack of the promised song via the Steam version. Well fear not, as it's already on your hard drive. You just have to know where to look.
Instructions after the word-portal, for those who don't want spoilers. (RSS readers: run away now).
You'll first need a smart little app called GCFScape, which can be found here. Once that's downloaded, install it, and let it associate itself with .gcf files.
Go into your Steam directory, and navigate to 'Steam\SteamApps'.
In there you'll see a file called 'portal content.gcf', which you can now double-click on. GCFScape will load, and show you two folders in the directory.
Make your way to portal - sounds - music, and there's the complete soundtrack for the game. That final song, that joyful, wonderful final song, is called portal_still_alive.mp3. Just copy the file to anywhere else on your computer, and listen away on loop for the rest of all eternity.
Then, make your way over to co-writer Jonathan Coulton's website, and spend some money on his songs.