Noisli is an app and browser-based background-noise generator as well as an ambient color background creator that “brings you the healthy benefits of the chromotherapy.” It allows you to mix and match different kinds of background noise at once. You can listen to rain, wind, water streams, a camp fire and thunder storms, and you can even make it as if you're traveling on a train to a remote client meeting.
The design is as beautifully simple as the concept: to increase productivity and relieve the anxiety of a tight deadline. It's used by coders, programmers, bloggers and writers I know and work with, and they all vouch for its efficacy.
I also use it everyday, especially in those "get shit done"moments. Coupled with a pair of noise-cancelling headphones, I find myself instantly more productive. It's also useful when delving into a new book or going to sleep, although I don't use it for this purpose as often as I'd like. The timer is also quite handy, allowing you to set a time to fade out the audio.
Simple concept, executed with the user in mind. Brilliant.