There are several Scheme implementations whose name is Scheme{number} or S{number}. This page aims to disambiguate them.
S2 was a Scheme-to-Scheme compiler by David Rush.
s7 is an interpreter for music applications by Bill Schottstaedt.
s7 is named after the Sunbeam motorcycle. It does not stand for Scheme 7.
Scheme 9 from Empty Space (aka Scheme 9, scheme9, s9, s9fes) is an interpreter by Nils M Holm.
The name of Scheme 9 was probably inspired by the operating system Plan 9 from Bell Labs, which in turn was named after the low-budget movie Plan 9 from Outer Space. A scheme is a plan.
Scheme 48 (aka scheme48, s48) is a seminal interpreter by Richard Kelsey and Jonathan Rees.
It is named thus because the authors wrote the rudiments of the implementation in 48 hours.
Scheme 84 was an implementation for Franz Lisp on VAX computers by Daniel Friedman.
Scheme 88 was a reimplementation of Scheme 84 on top of Ibuki Common Lisp.
Scheme 8088 (aka Scheme-88, s88) was an implementation for DOS.