S9fES is a mature, portable, and comprehensible interpreter for the R4RS Scheme programming language. It offers the following features:
The S9fES code strives to be simple and comprehensible. It is written in ANSI C (C89) mixed with Scheme and runs in many popular environments, including *BSD, Linux, Plan 9, and Windows.
S9fES has been placed in the public domain by its author. Because the author did not host the code for a while in 2011 and may or may not host it at any given time, this mirror has been established. Check out the original site  to see the current status.
The internal structure of the interpreter is documented in detail in the book "Scheme 9 from Empty Space" . An obsolete version of the book is available for free at .
You can download the S9fES source code from .