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.