s7 is a Scheme implementation intended as an extension language for other applications, primarily Snd and Common Music. It exists as just two files, s7.c and s7.h, that want only to disappear into someone else's source tree. There are no libraries, no run-time init files, and no configuration scripts. It can be built as a stand-alone interpreter (see below). s7test.scm is a regression test for s7. A tarball is available: ftp://ccrma-ftp.stanford.edu/pub/Lisp/s7.tar.gz.
s7 is the default extension language of Snd and sndlib (http://ccrma.stanford.edu/software/snd/), and Rick Taube's Common Music (commonmusic at sourceforge). There are X, Motif, Gtk, and openGL bindings in libxm (in the Snd tarball, or at ftp://ccrma-ftp.stanford.edu/pub/Lisp/libxm.tar.gz). If you're running s7 in a context that has getenv, file-exists?, and system (Snd for example), you can use s7-slib-init.scm to gain easy access to slib (this init file is named "s7.init" in the slib distribution).
Although it is a descendent of tinyScheme, s7 is closest as a Scheme dialect to Guile 1.8. I believe it is compatible with r5rs: you can just ignore all the additions discussed in this file. It has full continuations, dynamic-wind, sort!, error handling, ratios and complex numbers, define-macro, keywords, hash-tables, block comments, threads, multiprecision arithmetic for all numeric types, generalized set!, format, define*, and so on. It does not have syntax-rules or any of its friends, and it does not think there is any such thing as an "inexact integer".