iOS: There is a surprisingly long list of excellent ambient noise apps on the iPhone, but if you're looking for one that provides a nearly infinite amount of variety, White Noise+ is worth a look. White Noise+ is all about creating customisable sound effects. In this case, it features 80 sounds that you can place on a 4x4 grid. Each part of the grid effects the sound differently, making it softer, more complex, louder or simpler. The end result is a system where you can create an incredible number of ambient mixes. There are plenty of preset though if you're not interested in making your own. The app also comes packed with the basic utilities you'll need, including an alarm and sleep timer, iOS 10 widgets for quick playback and access to Airplay if you want to send the audio to speakers. You can check out a handful of sounds for free in an ad supported version, or unlock everything for $US2.99 ($4).
An app like Noisli is probably still best for those who prefer simplicity, but the intense amount of customisation in White Noise+ is worth a look if you've ever wanted to really create your own ambient noise.
White Noise+ (Free/$US2.99 [$4]) [iTunes App Store]