Stalin is an aggressively optimizing Scheme compiler. It is the most highly optimizing Scheme compiler, and in fact one of the most highly optimizing compilers of any sort for any language. Stalin is publicly & freely available, licensed under the GNU GPL. It was written by Jeffrey M. Siskind.
In detail, Stalin is a whole-program compiler that uses advanced flow analysis & closure conversion techniques. It compiles Scheme to highly optimized C. Stalin has a few very significant limitations, however:
The latest version is Stalin 0.11.
A humorous discussion about an excerpt from the stalin manpage can be found at gcc-does-no-flow-analysis.