myNoise sets the standard for online background noise machines and interactive soundscapes.
I've been listening to this site for about a decade and absolutely love it. It's made by a sound engineer who makes music, drones, white noise, field recordings, etc., sometimes with the cooperation of other artists. The site allows you to adjust a soundscape to your liking, and you can take individual audio tracks from multiple soundscapes to create your own composites, too.