Scheme48 is an implementation of Scheme written by Richard Kelsey and Jonathan Rees. It is based on a byte-code interpreter and incorporates such elements as a sophisticated module system, a pre-emptive & user-level thread system, a well-integrated interactive environment, Clinger's explicit renaming low-level macros, and decent efficiency for its architecture.
The current version of Scheme48 is 1.9 (release notes is at http://www.s48.org/1.9/release-notes.html). You can download it at http://www.s48.org/1.9/scheme48-1.9.tgz.
The Scheme Shell, Scsh (http://www.scsh.net), builds upon Scheme48 to provide an Unix shell embedded within Scheme. The current release of Scsh, 0.6.6, is based on version 0.53 of Scheme48.
Also kali scheme is based upon Scheme48. More information about kali is found on kali-scheme-revival
Scheme48 provides an interesting module system. Read a users guide to using it: scheme48-module-system.
Additional hacks and code specific to Scheme48 can be found on scheme48-hacks.