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

Yes, indeed.

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

No Scheme implementation is ultimately standalone. The bare-metal PC
schemes (Loko, Dream) and the microcontroller Schemes (Bit, PicBit,
ArmPit, Microscheme) may come closest. Most Schemes depend on C.  Kawa
and IronScheme depend on the JVM and the CLR, respectively. IMHO these
are prominent implementations by any reasonable measure.

These criteria are necessarily contentious. I still think it will be
best if we can agree on some and write them down. With transparency and
predictability we have the best shot at fairness which builds trust.
It's good to have the inevitable debates early.

A related point of contention is writing descriptions for the
implementations. https://get.scheme.org/ should have a dedicated page
for each (even the old and obscure ones). The descriptions should be
fair and balanced to both users and authors, which is not easy to achieve.

I once wrote some preliminary ones which are not yet linked to, e.g.:
https://get.scheme.org/chezscheme/ They seem to have been uploaded
accidentally, but should not yet be linked from anywhere. It would be
good to get feedback and improvements on these pages and get the
subdomain into a presentable state.l