Gerbil: R7RS, compiles to C, based on Gambit, extends gambit with better macro and module systems.
Racket: R6RS, beginner friendly, full Windows support, optional
typing, essentially a superset of scheme, tons of libraries, moving/moved to a Chez Scheme backend.
Transpilers to C
CHICKEN: R5RS and R7RS, beginner friendly, exceptional community, unique implementation of GC.
Cyclone: R7RS, experimental extension of Chicken-style GC with native thread support.
Gambit: R5RS, official installers also for
macOS, iOS, Windows, considered quite fast.
Bigloo: R5RS, can also compile to Java-Virtual-Machine (JVM) classes,
limited optional typing.
Gauche: R7RS, compiles to standalone
executable, official installers also for Windows, Docker.
GNU Guile: R6RS, getting JIT executable support soon, beginner friendly, officially supported by GNU, scripting language for many pieces of GNU software.
IronScheme: R6RS, based on Common-Language-Runtime (CLR).
Kawa: R7RS, based on JVM, compile to JVM classes, limited optional typing.
STklos: R7RS except for the module system; ad-hoc portable VM, with CLOS-like object system.
JavaScript Interpreter
BiwaScheme: R6RS, partial R7RS, compiler to intermediate represantion + VM
LIPS: R7RS, most of the spec is written in core Scheme, good interop with JavaScript