Chez Scheme is both a programming language and an implementation
of that language, with supporting tools and documentation.

As a superset of the language described in the
[Revised⁶ Report on the Algorithmic Language Scheme]
(http://www.r6rs.org)(R6RS), Chez Scheme supports all standard
features of Scheme, including first-class procedures, proper treatment
of tail calls, continuations, user-defined records, libraries,
exceptions, and hygienic macro expansion.

Chez Scheme also includes extensive support for interfacing with C
and other languages, support for multiple threads possibly running
on multiple cores, non-blocking I/O, and many other features.

WWW: https://cisco.github.io/ChezScheme

Thanks to jrm for helping with testing.