S9fES is a mature, portable, and comprehensible interpreter for R4RS Scheme. The core interpreter is written in pure ANSI C (C89), so it runs on virtually any platform offering a C compiler. It can even be compiled natively on Plan 9. On Unix-based platforms (plus Cygwin and MinGW), it also offers
The internals of the interpreter are discussed in detail in the Scheme 9 Book. [updated!]
Here is the syntax-highlighted interpreter source code and the man page.
S9fES has been developed using EDOC, a literate programming tool. However, the (rendered) EDOC source code is only available in print form. The C and Scheme code stripped from the EDOC files is in the public domain, though, and can be downloaded here:
The 2007 (1st Ed.) version is kept for those using the first edition of the book. If possible, use a later version.