It actually isn't a fork of Scratch3 but was started way back in Scratch1.4 as a JS implementation using Morphic which was inspired by Squeak. It is really good software and IMO is better than Scratch since it provides the tools to scale up from the toy programs Scratch is optimized for.
Snap! is made by folks previously involved in Berkeley Logo, and has a lot of "missing pieces" that make organizing programs easier: lambdas, cc, and binding functions to definitions (aka build-your-own-blocks).