RScheme  
   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]
   http://www.rscheme.org/

Pseudoscheme  
   Embeds Scheme in Common Lisp.
   http://www-swiss.ai.mit.edu/ftpdir/pseudo/

VSCM  
   A portable Scheme implementation written by Matthias Blume of Princeton University. No longer actively developed.
   http://www.cs.princeton.edu/~blume/vscm/

SCM  
   A portable Scheme implementation written by Aubrey Jaffer.
   http://www-swiss.ai.mit.edu/~jaffer/SCM.html

Gambit  
   Scheme system for DOS, Macintosh, Unix, Windows 32-bit, by Marc Feeley. Developed at University of Montreal. [Free to academic research, education]
   http://www.iro.umontreal.ca/~gambit/

Galapagos  
   An interactive multithreaded Scheme interpreter with turtle graphics for Windows 95, based on SCM.
   http://www.cs.bgu.ac.il/~elad/GALAPAGOS/

STk  
   A free R4RS Scheme interpreter which can access the Tk graphical package.
   http://kaolin.unice.fr/STk/

Elk (Extension Language kit)  
   (old site, last update in 1997) An embeddable, reusable extension language subsystem for applications written in C or C++. Elk is also a useful standalone Scheme implementation with interfaces to POSIX, Unix, and X11.
   http://www-rn.informatik.uni-bremen.de/software/elk/

Open Scheme  
   From Erian Concept, has CLOS-like object system; runs on Be/x86, FreeBSD, Linux/x86, Solaris (SPARC, x86), Windows. [commercial, unlimited free download evaluation copy with registration]
   http://www.open-scheme.com/

XLISP  
   A superset of the Scheme dialect of Lisp with extensions to support object-oriented programming.
   http://www.mv.com/ipusers/xlisper/

MIT Scheme  
   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 outpu
   http://www.swiss.ai.mit.edu/projects/scheme/

Larceny  
   A simple and efficient run-time system for Scheme, currently running on the SPARC architecture. A portable implementation that generates C (dubbed "Petit Larceny") is also being developed.
   http://www.ccs.neu.edu/home/will/Larceny/index.html

KSM-Scheme  
   A R5RS Scheme interpreter which integrates with C, allowing calling of C functions and accessing C variables from Scheme. It provides a mechanism to load C shared libraries. It runs on x86 and PowerPC based Linux systems.
   http://square.umin.ac.jp/~hchang/ksm/

QScheme  
   A fast, small Scheme interpreter which is mostly compliant with the R5RS standard.
   http://www.sof.ch/dan/qscheme/index-e.html

SXM, CXEMA  
   Portable Scheme implementation, conforms to IEEE/ANSI standard, supports all features of R5RS Report, many features of Chez Scheme and various SRFIs. Derived from XScheme: object system dropped, many changes, additions.
   http://www.malgil.com/sxm/

PC Scheme  
   Texas Instruments system for DOS OS; no longer maintained; download formats: source, executable/binary.
   ftp://swiss-ftp.ai.mit.edu:/archive/pc-scheme/

Indiana University Scheme Repository: Implementations  
   Even more Scheme implementations freely available from the Indiana U. repository.
   http://www.cs.indiana.edu/scheme-repository/imp.html

T  
   Scheme-like language developed at Yale; is to Scheme rather as NIL is to Lisp; mainly of interest to historians and theoreticians. FTP for v3.1.
   ftp://ftp.cs.indiana.edu/pub/scheme-repository/imp/t/

Scheme Implementations and Mailing Lists  
   A list of Scheme implementations from the Scheme FAQ maintained by Mark Kantrowitz.
   ftp://ftp.cs.indiana.edu/pub/scheme-repository/doc/misc/scheme_2.faq

Scheme 48 manual pages  
   HTMLified by Margaret Fleck, one of the authors of Envision.
   http://www.cs.hmc.edu/~fleck/envision/scheme48/user-guide.html

3DScheme  
   Two systems for Windows with built in "industrial-strength" solid modeler based on the ACIS solid modeler Geometric Modeling Kernel.
   http://www.schemers.com/3dscm1.html

EdScheme  
   Interpreter for Macintosh or Windows. Has "friendly and convenient interactive programming environment" with language sensitive editor, full documentation. [commercial, free 14 day download]
   http://www.schemers.com/edschem.html

The Ksi Scheme Interpreter  
   Ksi is a portable, embeddable Scheme implementation written in C. Unfortunately its documentation is all in Russian at this point.
   http://ksi.sourceforge.net/

SIOD: Scheme in One Defun/Day  
   Very small, portable implementation, has some database, Unix programming, CGI scripting extensions. Runs on DOS, Linux, Unix, Windows. Free source downloads.
   http://people.delphiforums.com/gjc/siod.html

Scheme 48  
   A small and portable implementation based on a bytecode interpreter designed to be used as a testbed for experiments in implementation techniques.
   http://s48.org/

TinyScheme  
   A tiny implementation of Scheme based on MiniSCHEME. It provides almost complete coverage of R5RS Scheme. Geared towards embedded scripting use, but also functions as a standalone interpreter and extensible shell tool. Recent changes have gotten the exe
   http://tinyscheme.sourceforge.net/

LispMe  
   Fred Bayers home page is really the home of this Scheme for 3COM Pilot PDA systems: compiler and runtime system intended mainly as a tool to quickly try ideas and algorithms, but can write dialog-based applications.
   http://www.geocities.com/SiliconValley/Lab/9981/main.html

STklos  
   Derived from STkn implementation based on an ad-hoc virtual machine, byte compiler; compilable as library to embed in other programs, has object system with MOP, multiple inheritance, generic functions, multimethods, module system, full R5RS tower of numb
   http://kaolin.unice.fr/STklos/

Scsh FAQ  
   The FAQ for Scsh, the Scheme Shell.
   http://www-internal.alphanet.ch/~schinz/scsh-faq/scsh-faq_toc.html

Scsh  
   A broad-spectrum systems-programming environment for Unix embedded in R5RS Scheme (actually within version 0.53 of Scheme48). Support for concurrent system programming, sophisticated I/O and automatic garbage collection for process resources.
   http://www.scsh.net/

Scheme->C  
   DEC's venerable Scheme to C translator which runs on most anything with an ANSI C compiler.
   http://ftp.cs.indiana.edu/pub/scheme-repository/imp/Scheme-to-C/

Vx-scheme  
   A compact (<64 Kb) implementation of R4RS Scheme that compiles for VxWorks/Tornado (and is integrated with the system symbol table). [Open source, Artistic License]
   http://colin-smith.net/vx-scheme/

WinScm  
   Scheme environment for Windows 3.1/95; interpreter independent, though defaults to Jaffer's SCM. Used at University of Lille 1, France, for introductory programming course. Free downloads. English, Française.
   http://www.lifl.fr/~routier/enseignement/winscm/winscmeng.html

Hotdog  
   A nearly R5RS compliant Scheme compiler with multiple backends, currently including .NET, JVM, and C. [Public Domain]
   http://rover.cs.nwu.edu/~scheme/

Infer Project  
   Statically-typed Scheme dialect, written in Infer, combines many of the best features of Scheme and ML. NSF funded.
   http://www.cs.indiana.edu/~chaynes/infer.html

Stalin  
   A powerful optimizing Scheme compiler from Jeffery Mark Siskind at the NEC Research Institute. Sacrifices functions such as call/cc in favor of efficiency, but generated code is remarkably bulletproof and fast.
   http://www.ece.purdue.edu/~qobi/software.html

DrScheme  
   Full Scheme programming environment for Macintosh, many Unixes, Windows. Has module and object systems, platform-independent graphics. Ideal for beginners; one of the major design goals is a flexible teaching environment.
   http://www.drscheme.org/

CMU AI Repository Free/Shareware Scheme Implementations  
   Yet more Scheme implementations from the CMU AI Repository. Many of these are also available from the Indiana University Scheme Repository.
   http://www-2.cs.cmu.edu/afs/cs/project/ai-repository/ai/lang/scheme/impl/0.html

Pocket Scheme  
   For MIPS, SH3, ARM-based Windows CE devices. Supports Aubrey Jaffer's SLIB, has initialization file for same, parenthesis-balancing text editor. Windows NT version exists. [Open Source, Artistic License]
   http://www.mazama.net/scheme/pscheme.htm

Inlab Scheme  
   A commercial Scheme which is freely available for non-commercial use on Linux and FreeBSD. It has support for several features like bitmap/greymap processing and can be used as a general tool for image processing, OCR or specialized optical object and pa
   http://www.inlab.de/scheme/

MzScheme  
   Small, embeddable, scripting PLT Scheme implementation: R5RS compliant, full numerical tower, threads (all platforms), exceptions, modules, class-based objects, regular-expression matching, TCP/IP. For Macintosh, Unix, Windows 32-bit.
   http://www.plt-scheme.org/software/mzscheme/

PLT Scheme  
   Advocacy and authoring group, umbrella name for family of implementations: DrScheme (Scheme, MzScheme, MrEd); books: How to Design Programs, How to Use Scheme; and TeachScheme! project. University-based, supported by many volunteers writing code, document
   http://www.plt-scheme.org/

Bigloo  
   System with one goal: enable Scheme-based programming style where C(++) is usually needed; makes Scheme practical via features found in most traditional languages but not Scheme and functional programming. [Open Source, GPL]
   http://www-sop.inria.fr/mimosa/fp/Bigloo/

Better Scheme  
   Language designed and largely implemented, now a matter of coding, documenting. Goals: high consistency; improve language's functional nature; consistency with lambda calculus; optimize, but not at cost of other goals.
   http://www.cs.oberlin.edu/~jwalker/bscheme/

Sizzle  
   A Scheme interpreter implemented as a library which can be embedded into C programs, as well as a standalone interpreter. Mostly R5RS, Guile compatible, and includes regular expressions and most Posix functions.
   http://uebb.cs.tu-berlin.de/~magr/sizzle/sizzle.en.html

narsi  
   A portable scheme interpreter by Britt Park. Also other free software, including: Cdoc - a simple program to generate pretty TeX output from annotated c or c++ code; Userblock - a simple user space block device driver kit for Linux; UVFS - a user space fi
   http://www.sciencething.org/geekthings/

Elk Scheme - the Extension Language Kit  
   (new site) An embeddable, reusable extension language subsystem for applications written in C or C++. Elk is also a useful standalone Scheme implementation with interfaces to POSIX, Unix, and X11.
   http://sam.zoy.org/projects/elk/

Chicken  
   A Scheme compiler which compiles a subset of R5RS into C. Uses the ideas presented in Baker's paper "Cheney on the MTA". It is highly portable, reasonably efficient and makes interfacing to C and C++ very easy.
   http://www.call-with-current-continuation.org

Gauche  
   An R5RS Scheme implementation developed to be a handy script interpreter. Quick startup, built-in system interface, native multilingual support. [Open source, BSD license]
   http://www.shiro.dreamhost.com/scheme/gauche/

Schoca (Scheme for OCaml)  
   An implementation of the Scheme language in OCaml. The primary purpose of Schoca is the use as an embedded extension language in OCaml applications. [Open source, GPL]
   http://home.arcor.de/chr_bauer/schoca.html

Kali Scheme  
   A distributed implementation of Scheme that permits efficient transmission of higher-order objects such as closures and continuations.
   http://community.schemewiki.org/kali-scheme/

Chez Scheme  
   A Scheme implementation. [Commercial]
   http://www.scheme.com/chezscheme.html

Petite Chez Scheme  
   A complete Scheme system that is fully compatible with Chez Scheme but uses high-speed threaded interpreter technology in place of Chez Scheme's incremental native-code compiler. [Freeware]
   http://www.scheme.com/petitechezscheme.html