[smcameron]

There's OpenSimplex by Kurt Spencer. I made a port of his original java implementation to C a few years ago.

https://github.com/smcameron/open-simplex-noise-in-c

Edit:

Here's the original: https://github.com/KdotJPG/OpenSimplex2

[mooman219]

It's worth noting that OpenSimplex has different characteristics to Simplex in higher dimensions. Specifically some have noticed a stark tendency to form orbs in 4D, so it's definitely worth comparing the output of all 3 (perlin, simplex, and OpenSimplex) to see if they give you what you're looking for.

Additionally, it may or may not be legally distinct (hasn't been tested).