Small, standard-complying implementation, in C++. [Open source, zlib license]
Portable, extended Scheme with reflective object(-oriented) system, operating system services, modules, threads, many system programming features (integrates with, compiles to C or bytecodes) and useful extensions. [Open Source]
Embeds Scheme in Common Lisp.
A portable Scheme implementation written by Matthias Blume of Princeton University. No longer actively developed.
A portable Scheme implementation written by Aubrey Jaffer.
A portable, high-performance implementation of Scheme based on an optimizing compiler by Marc Feeley. Developed at University of Montreal.
An interactive multithreaded Scheme interpreter with turtle graphics for Windows 95, based on SCM.
A superset of the Scheme dialect of Lisp with extensions to support object-oriented programming.
From MIT, for many systems, has comprehensive code library, includes most functionality of ANSI Standard Common Lisp (CLtL2), many low-level OS interactions. Distributed with system is LIAR (LIAR Imitates Apply Recursively), optimizing compiler that outputs native machine code, has Edwin interactive Emacs-derived editor written in pure Scheme and the subject of an MIT AI Lab Memo. Arguably one of the best Scheme systems available, unarguably one of largest. Major downfall: its not totally R5RS compliant, more so for hygienic macros and #f versus the empty list. Version 7.5 now available for x86 systems.
Simple, efficient Scheme run-time system, for SPARC architecture. Petit Larceny is portable and emits C, for Linux (Intel IA32), Macintosh OS X (PowerPC), Solaris (SPARC), Windows. Common Larceny runs on Microsoft Common Language Runtime (CLR), emits MS IL not native or C code.