stegu/webgl-noise is a repository of GLSL shaders implementing Simplex, Perlin, and Worley noise.

The library includes a variant of Simplex Noise that supports rotation within each hexagonal area. Watching the animation shows how Simplex Noise results in hexagons divided into a dark and light half:

The library includes periodic Simplex (2D) and periodic Perlin noise (2D, 3D, 4D).
per.x= per.y=

Some of the shaders also include variants that calculate analytic derivatives.

Click on an image to generate a shadertoy-compatible shader: