Scheme 48 - It's called “Scheme 48” because the first version was written in 48 hours. Scheme 48 is an implementation of the Scheme programming language as described in the R5RS.
SCM - SCM is a Scheme implementation conforming to R5RS and the IEEE P1178 specification.
Gambit - The Gambit-C system conforms to the R4RS, R5RS and IEEE Scheme standards and implements all optional features.
Guile - Guile implements Scheme as described in the R5RS, providing clean and general data and control structures.
Gauche - Gauche is an R5RS Scheme implementation developed to be a handy script interpreter, which allows programmers and system administrators to write small to large scripts for their daily chores.

Chicken - CHICKEN produces portable, efficient C, supports almost all of the R5RS Scheme language standard, and includes many enhancements and extensions.
KSM-Scheme - KSM-Scheme implements R5RS standard. In addition to the standard, KSM-Scheme has extensions that enable to call C functions and to manipulate C variables.

QScheme - QScheme is a small and fast Scheme interpreter. It's mostly R5RS conformant.
RScheme - RScheme is an implementation of the Scheme language, with it's object-orientation approach adapted from the Dylan language. RScheme implements all of R4RS except transcript-on and transcript-off, and all of R5RS except define-syntax.
TinyScheme - TinyScheme is a lightweight Scheme interpreter that implements as large a subset of R5RS as was possible without getting very large and complicated.

PLT Scheme - PLT Scheme tools in their default modes do not conform to R5RS, mainly because PLT Scheme tools generally expect modules, and R5RS does not define a module system. PLT Scheme’s R6RS support also does not conform with the standard in several known ways.
Ikarus - The current release of Ikarus supports over 94% of the most important features R6RS, and later releases will bring Ikarus closer to full R6RS conformance.

Chez Scheme - Chez Scheme is an efficient and reliable implementation of Scheme based on an incremental optimizing compiler that produces efficient code and does so quickly. Chez Scheme Version 8.0 is an implementation of R6RS Scheme along with numerous language and programming environment extensions.

Learning

Training Courses

Structure and Interpretation of Computer Programs
- SICP Wiki - This wiki is dedicated to the wonderful book “Structure and Interpretation of Computer Programs”. It contains solutions to exercises along with related discussions, useful links, etc.
Concrete Abstractions - An Introduction to Computer Science Using Scheme.