Also known as Cell Noise or occasionally Voronoi Noise, this is a texture is built by computing the distance to randomly distributed points, and weighting the lightness of the each pixel by the distance from the closest point.
PCG Wiki References
Images on Flickr
The following images on Flickr have been generated using Worley Noise:
Worley_Noise - Wikipedia article on Worley Noise.
Voronoi / Worley Noise Knowledge Dump - Comprehensive article on Worley Noise including Python code.
A Good Tutorial on Worley Noise - A tutorial on Worley Noise with a demo and source code.