iOS: We're fans of the Noisli web app that allows you to chain together different background sounds to create your own little productivity bubble. Now, the web app is available on the iPhone.
The Noisli iPhone app allows you to play a selection of sounds, including rain, thunderstorms, a stream, bonfire and plenty of others. You can play as many different sounds as you want at once and save your favourite combos to play later. The app also has a timer function so that it will turn off after a certain amount of time. The experience is similar to the web app, but everything is available offline, so you don't need to worry about a connection.
Noisli ($1.29) [iTunes App Store]