Noisli - Noisli is a fantastic background noise and color generator for working and relaxing. Online soothing ambient sounds like White noise, Rain and Coffee Shop.
Noizio - Feeling that the noise of the big city is too much for you to bear?
White Noise - White Noise to help you sleep and games to wake you up.
zenmix.io - Ambient background sound generator
Brain.fm - Music designed for the brain to enhance focus, relaxation, meditation, naps and sleep within 10 - 15 minutes of use.
Hear - Hear is a sound enhancement application for the Mac.