I recently discovered that the sound of rain makes a great ambient sound for working. Unlike with real music I do not have to switch artists or albums every few hours and there are no foreign words injected into my thinking process so it's easier for me to focus. Finding a good recording of rain can be a bit difficult. What I ended up using is these two CC-BY-licensed samples in loop at the same time :
The Freesound website makes that kind of playback possible without even involving sound editors. All you need is two tabs (or even one) kept open in a browser.