A friend of my showed me great web site recently – Noisli.com. It allows to compose your own set of sounds that will help you tune out, focus and get in the zone. There are various sounds to choose, like: rain, wind, water, bonfire, etc. You can pick few and tune the volume of each. I personally, find very pleasant a combination of rain, thunderstorm and bonfire.
If you have a spare minute I strongly advise to give Noisli.com a try.