Whilst not music, I find listening to http://www.rainymood.com to be great for concentrating. It's a 30 minute sample of a rain storm.
It got me through my Master's dissertation, where I had it on loop for hours at a time.
Fiddler tells me you can currently grab the mp3 from here http://173.193.205.68/audio/RainyMood.mp3 (this was especially useful for the times I wanted to be disconnected from the internet to focus on work).