The most comprehensive list of Scheme implementations is in the scheme-faq .
Here is a quick-and-dirty list of a very select number of Scheme implementations.
Compiles Scheme to C, JVM and .NET.
A very fast cross-platform native code compiler (commercial) along with a free interpreter.
A very small library intended for use as an extension and scripting language in C programs.
A Scheme->C compiler with a good FFI?.
A brand-new Scheme-to-C compiler that allows practical application development using R7RS Scheme.
A Scheme based extension language kit.
This wiki runs on it. A scripting language scheme interpreter with good Unicode support.
The GNU Scheme interpreter, designed to have good integration with C programs. A nice extension language.
R6RS Scheme incremental compiler to i32 and i64 native code.
R6RS Scheme for the .NET Framework.
A variant of Scheme48 for distributed computation.
Runs on the JVM
Is a simple and efficient R6RS runtime for scheme.
R4RS Scheme, but for Palm Os
An R5RS Scheme interpreter in OCaml.
A purely functional dialect of R7RS Scheme. Small, embeddable, cross-platform. Has own FFI.
A new implementation currently in prerelease stage.
PLT Scheme is an umbrella name for a family of implementations of the Scheme language - DrScheme, MzScheme, MrEd.
A scheme for WinCE based handhelds.
Compiles to C, has Dylan-like OO features.
The primary design goal was clarity and good design.
A Scheme interpreter in Objective Caml.
Small and portable R5RS implementation. SLIB, by the same author, is nicelly integrated with SCM.
The SCheme SHell, based on Scheme48. This adds a lot of very interesting facilities for POSIX system interfacing.
A Scheme interpreter in Java, making the whole Java library available to Scheme, and still being very fast (for an interpreter)
The framework for creating web applications using the SISC interpreter.
Scheme to C compiler
A Scheme System with an Object System and TK integration
Is the name for an implementation and dialect of scheme
A lightweight Scheme interpreter
An embeddable C++ implementation