Akku.scm is a language package manager for R6RS Scheme.
Loko Scheme is an optimizing R6RS Scheme compiler for Linux, NetBSD/amd64 and bare metal. It can be used to build operating systems in Scheme.
Industria is a collection of portable R6RS Scheme libraries for cryptography. It implements low-level algorithms like AES, RSA, DSA, ECDSA, etc and high-level protocols such as OpenPGP, Off-The-Record messaging, DNS and Secure Shell (SSH).
The GNU Guix project found the OpenPGP implementation useful for quickly validating signatures using a pre-validated keystore.
machine-code is a set of tools for working with machine code (disassemblers, an x86 assembler, ELF format, stuff like that).
The disassembler was useful to the Racket-on-Chez effort.
r6lint is a linter for R6RS Scheme. The idea is to help the programmer write better code by checking the code on the fly in the editor.
Zabavno is an x86 emulator in R6RS Scheme.
The Erlang Supercompiler is currently on my gitweb.