Atmosphere Lite is an interesting tool for generating nature-based ambient sounds. In a click or two the program can take you to a forest stream, a woodland campfire, the ocean's edge and more, complete with any associated sounds: waves, wind, rain, birdsong and more.
The program isn't just replaying a single audio file. It works by using multiple sounds samples, blending them together, and launching them at random times and frequencies. The end result is that even something relatively simple - the "Night Stream" scenario, say, is mostly just gurgling water and a few owl noises - can play in the background for a very long time, without you getting bored or noticing any repetition.
If you don't find the built-in sound scenarios interesting, though, they can be reconfigured in various ways. New background sounds (rain, wind, stream, ocean) may be added with a click, there are plenty of random sounds to include (thunder, birds, even insects), and you can play around with volume - making background sounds a little quieter, say - to help produce the effect you need.
The end result is surprisingly effective, and can be genuinely relaxing. To help a little more, Atmosphere Lite is able to display a full-screen slideshow while you listen. And if you don't want to risk drifting off to sleep - or at least, not for long - there's an option to set an alarm.
As you'll probably guess from its name, Atmosphere Lite is the free version of a commercial program, and upgrading to the PLUS or DELUXE editions will get you many more sounds, scenarios and slideshows. These each support more variations - there's "dynamic weather" and "evolving soundscapes" - and you also have more options for fine-tuning the results, including the ability to tweak the volume of individual sounds. Prices start at $22.95 and you can find out more at the developer's site.