Moodwill - Create and listen for free to your perfect nature sounds environment with music to help you concentrate, relax, sleep and overall feel better.
Plum Village - Free mindfulness app w/ Zen Buddhist meditations + teachings
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.
Calm - Calm.com can help you reduce stress and increase calm.
Silenz - Mix ambient noise in with the music you're listening to
Headspace - Meditation made simple. Brilliant things happen in calm minds.