Recently, there have been several releases of non-mainstream Scheme systems (for whatever value mainstream has in the context of Scheme implementations). All of them have a long history and are developed by people high up in the community’s iconography. They are also very portable and, hence, you’ll probably be able to play with them, learn and take advantage of their unique features (both as a programmer and as an implementor: the sources are there in the open too).
The Gambit Scheme System, by Marc Feeley, includes an interpreter and a compiler using C as intermediate language. Among many interesting features (and the coolest logo), it has the ability of producing standalone executables, and often requested feature. (I’m personally more interested in how good are its interpreter and module system at providing a truly dynamic development environment, but that’s another story.) Gambit features an extremely efficient thread system, capable of supporting millions (sic) of concurrent processes. Another nifty feature are readtables, akin to Common Lisp’s reader macros, making it an excellent choice if you’re into the parsing business or simply want to have a little extra fun. My only nit (and the reason i’m not trying to include Gambit support into Spells) is that the support for SRFIs is relatively weak: are you looking for an interesting and fairly doable hacking project?.
Update: As it comes out, it’s better than i thought. One of the Gambit developers has pointed me to this quick and dirty, yet convenient hack that, when run in Gambit’s installation directory, will automagically install the indispensable SRFI-1.
Stklos also released a new version this January. After being dormant for some time, development seems to progress at full steam lately. Stklos is the successor of Stk, initiated by Erick Gallesio in the early 90s, and was pointed out by RMS as “the Tcl substitute” in his famous Why you should not use Tcl flamewar. Unfortunately, the FSF later chose Guile as the Tcl substitute, instead of this very interesting implementation, whose current incarnation includes some unique offers. To begin with, Stklos features an efficient and powerful object system based on CLOS, another of my CL-envies. Also of note is that it comes with Gtk+ integration: follow the link to see some nice screenshots with source code. Personally, and with an eye on the s48-worlds project, i also find interesting its ability to install extensions downloaded from public repositories. Finally, its SRFI support is really excellent. On the downside, well, its logo is not so cool, is it?
The Larceny Project has just announced their Operation Drop-Kick release of Petit Larceny, a portable Scheme to C compiler which now supports OS X, GNU/Linux, Windows and Sparc/Solaris. The Larceny project was initiated by Will Clinger back in 1991, and has a strong focus on garbage collection and compiler optimisations. I have not really used it, but my uninformed opinion is that Larceny will be mainly useful to those of you doing research (or with a strong interest in) those areas.