Based on XLisp, but with built-in statistics support.

Lisp version based on, and evolutionary improvement on, experimental XLisp.

A compact (<64 Kb) implementation of R4RS Scheme that compiles for VxWorks/Tornado (and is integrated with the system symbol table). [Open source, Artistic License]

Small, standard-complying implementation, in C++. [Open source, zlib license]

Based on Mini-Scheme, tiny implementation, almost a full R5RS Scheme. Goal: embedded scripting use, but works as standalone interpreter and extensible shell tool. Recent changes reduced executable size to about 64KB on Linux/x86. [BSD]

Ksi is a portable, embeddable Scheme implementation written in C. Unfortunately its documentation is all in Russian at this point.

History with: downloads, contributors, timeline, publications, influence, users, successors, Olin history errata. By Jonathan A. Rees.

Fairly detailed, by Olin Shivers.

Portable Scheme implementation, conforms to IEEE/ANSI standard, supports all features of R5RS Report, many features of Chez Scheme and various SRFIs. Derived from XScheme: object system dropped, many changes, additions.

Very small, portable implementation, has some database, Unix programming, CGI scripting extensions. Runs on DOS, Linux, Unix, Windows. Free source downloads.