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.
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.