There's something oddly relaxing about listening to the sound of rainfall - an opinion a lot of people share. With that in mind, the internet is home to a large number of rain noise generator websites. One of the best is rain.today, created by sound designer Stéphane Pigeon.
The site has four modes: Light Rain, Heavy Rain, Thunder, and "After the Rain" (a sound like water streaming along the ground). The generator slowly morphs through each of these based on a type of random selection algorithm but you can turn specific modes off if you don't want to hear, say, thunder. Each mode consists of dozens of individual recordings that seamlessly blend into each other, resulting in a very natural, realistic experience.
The site is currently in beta, but new features are in the pipeline, as are apps for iPhone and Android.