These are implementations that would make sense to look into for your own commercial, production applications.
I don't know whether to put Microsoft's ChakraCore into this list or the next. So I'll put it here but note that as of this year 2021, they are transitioning it to become a community-driven project.
Implementations toward the top are more reliable and proven. Implementations toward the bottom less so.
If you are a looking to get involved in language development, the implementation further down on the list can be a great place to start since they typically need work in documentation, testing, and language features.
Thanks to @smarr for contributing eJS, Higgs, and b9!
Great for inspiriration if you've never implemented a language before.