Ur-Scheme is a compiler from a small subset of R5RS Scheme to Intel x86 assembly language for Linux. It can compile itself. It is free software, licensed under the GNU GPLv3+. It might be useful as a base for a more practical implementation (or a more compact one), or it might be enjoyable to read (particularly since the entire development history is browsable using darcs), but it probably isn't that useful in its current form.
Influences and Inspirations に挙げられている資料は一読したいところ。