Scheme 48 is a free software Scheme implementation using a bytecode interpreter. It has an foreign function interface for calling functions from C and comes with a regex library, and a POSIX interface. It is supported by SLIB the portable Scheme library and is the basis for Scsh, the Scheme shell.
It's called "Scheme 48" because the first version was written in 48 hours. The authors now say it is intended to be understood in 48 hours.