S9fES is a mature, portable, and comprehensible R4RS Scheme interpreter with an (optional) Unix interface. It is in the public domain. You can download it here (~450KB).
total 876 -rw-r--r-- 1 nmh nmh 1017 Oct 3 17:17 ABOUT -rw-r--r-- 1 nmh nmh 19701 Jan 10 09:20 CATEGORIES.html -rw-r--r-- 1 nmh nmh 70754 Jan 9 19:23 CHANGES -rw-r--r-- 1 nmh nmh 1111 Nov 24 13:24 LICENSE -rw-r--r-- 1 nmh nmh 8633 Nov 11 10:59 MASCOT.png -rw-r--r-- 1 nmh nmh 8759 Dec 6 10:10 Makefile -rw-r--r-- 1 nmh nmh 7797 Jan 3 09:47 README -rw-r--r-- 1 nmh nmh 203 Jan 10 09:19 Todo -rw-r--r-- 1 nmh nmh 28699 Jan 10 09:19 _checksums -rw-r--r-- 1 nmh nmh 654 Jul 30 2010 config.scm -rwxr-xr-x 1 nmh nmh 629 Jan 8 10:04 configure drwxr-xr-x 2 nmh nmh 1024 Jan 10 09:18 contrib drwxr-xr-x 2 nmh nmh 1024 Oct 11 2010 ext drwxr-xr-x 2 nmh nmh 512 Oct 6 09:14 freebsd-port drwxr-xr-x 2 nmh nmh 15872 Nov 27 16:35 help drwxr-xr-x 2 nmh nmh 3584 Dec 29 18:58 lib -rw-r--r-- 1 nmh nmh 161 Dec 9 2008 mkfile drwxr-xr-x 2 nmh nmh 512 Nov 25 09:56 prog -rw-r--r-- 1 nmh nmh 16897 Nov 12 11:38 s9-real.c -rw-r--r-- 1 nmh nmh 17337 Oct 13 09:53 s9-real.scm -rw-r--r-- 1 nmh nmh 15372 Nov 27 16:29 s9.1 -rw-r--r-- 1 nmh nmh 17764 Nov 27 16:30 s9.1.txt -rw-r--r-- 1 nmh nmh 114424 Jan 9 11:09 s9.c -rw-r--r-- 1 nmh nmh 12902 Nov 11 18:01 s9.h -rw-r--r-- 1 nmh nmh 21504 Nov 11 18:11 s9.scm -rw-r--r-- 1 nmh nmh 456790 Jan 10 09:20 s9fes.tgz drwxr-xr-x 2 nmh nmh 512 Nov 15 09:12 util