LibNoise.NET
A Fork (with nuget support) of the seemingly (approx. four years) dead LibNoise.NET.
Here's the full previous description, for posterity:
A C# port of libnoise (http://sourceforge.net/projects/libnoise/), a general-purpose library that generates three-dimensional coherent noise.
Many thanks to gharen for creating this project, and licensing it under the MIT License, permitting me to keep it somewhat up to date 😄