Scheme2Js is a Scheme to Javascript compiler distributed under the GPL license. While some effort has been spent on being as close as possible to R5rs, we concentrated mainly on efficiency. Usually Scheme2Js produces Javascript code, that is comparable to
An interesting tool for numerical computation and data analysis, based on OCaml. Its author, David McClain designed the Lisp-based Siglab system afterwards. Interfaces exist for C/C++, VBasic, OCaml, Dylan, Mathematica, Lisp, and Scheme.
Marc Feeley demonstrated how to write a simple Scheme to C compiler, in Scheme. In only 90 minutes! Presentation material (slides?), source code and video (200MB) are available.
spmr, a Scheme replacement for procmail. This mail delivery agent lets you sieve and handle incoming files with the comfort of Scheme sounds interesting.