This is a C# implementation of the Perlin simplex noise, an improved Perlin noise algorithm. It was (loosely) based on SimplexNoise1234 by Stefan Gustavson, a library released to the public domain under the GNU General Public License.
This implementation is "Simplex Noise" as presented by Ken Perlin at a relatively obscure and not often cited course session "Real-Time Shading" at Siggraph 2001 (before real time shading actually took on), under the title "hardware noise". The 3D function is numerically equivalent to his Java reference code available in the PDF course notes.