Moodwill - Create and listen for free to your perfect nature sounds environment with music to help you concentrate, relax, sleep and overall feel better.
Sounds of Earth - Listen to relaxing ambient sounds of nature
zenmix.io - Ambient background sound generator
Noizer - Ambient sounds like Noisli, but free
Defonic - Create a unique sound environment at home or in the office
Ambie - Beautiful white noise app to help focus, sleep, or meditate.