Scheme 9 from Empty Space is a work in progress. It will eventually
become an interpreter for a broad subset of R5RS Scheme. Its core is
written in ANSI C (C89).

The S9fES code strives to be comprehensible rather than fast. It is
aimed at people who want to study the implementation of Scheme (in a
language other than Scheme).

The S9fES homepage is here: http://t3x.org/bits/s9fes

The interpreter is not really useful at this stage, but it is making
continuous progress. Currently it supports:

Syntax: and, begin, define, if, lambda, let, letrec, or, quote, set!.
Procedures: *, +, -, <, apply, car, cdr, cons, eq?, quotient, remainder.

It already does garbage collection and bignum arithmetics.

The meta-project would be to develop really nicely coded
Scheme implementations, each of which hit a distinct
sweet-spot in the design space.   E.g., one implementation
that aims to be the smallest (an SIOD follow-up).   Another
that aims to be the hottest pure interpreter (an SCM
replacement).   Another that aims to be the tightest effective
to-C compiler.   Another that aims to be an optimizing
to-C compiler.  Another that aims to be an easilly retargetted
native compiler.   Factor JIT into the list.  Etc.

The meta-meta-project would be to call out what those
sweet-spots are (the list above is speculative and I'm
sure I'd change it if called upon to commit to one).
A meta-meta-meta project might be to try to float a common
run-time system out of all that so that extensions in languages
other than Scheme would have a common target for "language
bindings" to Scheme.