celestial (Free|$1.99, £1.49, AU$2.49) is an ambient noise app that claims to help you relax or focus, and it has a timer so you can listen while you fall asleep. Note that the free version comes with three sounds, and you can unlock three more via an in-app purchase, but if you buy the paid version you can skip this step.
The full version gives you six different ambient soundscapes that were recorded at real locations around the globe. Launch the app, select a sound type, press Play and melt away into your work or thoughts. Some design missteps that make the interface a bit confusing, but the sounds are of high quality, making the app worth your while.