The wiki for this repository contains more information.

Simplex noise functions are (C) Ashima Arts and Stefan Gustavson
Classic noise functions are (C) Stefan Gustavson
Cellular noise functions are (C) Stefan Gustavson
The "psrdnoise" functions are (C) Stefan Gustavson

Source code for the noise functions is released under the
conditions of the MIT license. See the file LICENSE for details.

The simplex noise functions follow Ken Perlin's original idea,
more clearly explained in Stefan Gustavson's paper
"Simplex noise demystified"
http://www.itn.liu.se/~stegu/simplexnoise/simplexnoise.pdf
but without using any uniform arrays or texture engines.

Many other noise implementations make heavy use of a
texture lookup table and are texture bandwidth limited.
The noise functions in this library, however, are completely
self contained with no dependency on external data.
While not quite as fast as texture-based implementations
on typical current desktop GPUs, they are more scalable to
massive parallelism and much more convenient to use, and
they can make good use of unused ALU resources when run
concurrently with a typical texture-intensive rendering.

2016-05-13: Ashima Arts now seems to be defunct as a company
(their website and email addresses have ceased to function)
so I cloned this repository to:

https://github.com/stegu/webgl-noise/

This site is heavily linked from all over the place on the
Internet, so I (Stefan Gustavson) will keep updating both sites
for the foreseeable future as the (apparently) sole maintainer.