There is also LIPS, but it doesn't seem to fully support any RnRS yet
and has no SRFIs.

https://lips.js.org/

I'm not sure it would count as an standalone implementation, since it is
tied-in to the Javascript environment.

Lassi Kortela <xxxxxx@lassi.io> writes:

> Here's the current list of maintained implementations as far as I can tell:
>
> Bigloo
> Chez Scheme
> Chibi Scheme
> CHICKEN
> Cyclone
> Gambit
> Gauche
> Gerbil
> Guile
> JazzScheme
> IronScheme
> Kawa
> Loko
> MIT Scheme
> Mosh
> Racket
> s7
> Sagittarius
> SCM
> STKlos
> TinyScheme
> Ypsilon
>
> There are some new upstarts on GitHub and GutLab that I'm not familiar with.