I Dose has a collection of free sounds that, they say, can help relax your stressed brain or make it more alert and improve the concentration level.
These are basically Binaural Beats and used by neurophysiologists to lower the brain frequency to relax the listener or to raise it to help focusing.
I haven’t tried them but there’re a few interesting tones. For instance, i you are addicted to caffeine, try the Coffee Break tone as a replacement for drinking coffee. Then there’s an Inspiration tone for creative artists and writers.
Related: Free MP3 Music for Podcasts