Chicken is a Scheme compiler that translates most of R5RS Scheme into relatively portable C. It supports fully general tail-call recursion, first-class continuations, and has a very flexible and efficient interface to C and C++. Chicken implements several extensions to the Scheme language: lightweight threads, pattern matching macros, dynamic loading of compiled code, and various object-oriented paradigms, such as TinyCLOS, and others. The library system includes hundreds of convenient modules for practical use.
Gauche is an R5RS Scheme implementation that aims to be a handy tool for daily work. Quick startup, a built-in system interface, and native multilingual support are some of its goals. It has an OO system similar to STklos and Guile. It supports UTF-8, EUC-JP, and Shift-JIS multibyte encodings natively.
Geiser is a generic Emacs/Scheme interaction mode, featuring an enhanced REPL and a set of minor modes that improve Emacs' basic major mode for Scheme. The main features provided are evaluation of forms in the namespace of the current module, macro expansion, loading of files and modules, namespace-aware identifier completion, automatic documentation, jumping to the definition of an identifier, access to documentation, listings of identifiers exported by a given module, and rudimentary support for debugging. Geiser supports Guile and Racket.
Kahua is a continuation passing style (CPS) application framework and server. It is written using the Gauche implementation of Scheme. It has the advantage that you can use "continuations" easily, so you can solve the typical Web application's problem of process disconnection. It allows you to program declaratively by using S-expressions as a fundamental data type, you can use Kahua's Object Database to persist data, and you can modify the function being run, supporting incremental development.
Kipt (Kizome's ISA Piercing Tool) is an HTTP proxy server and proxying name server with the ability to forward requests to other HTTP proxies or to get the documents directly from hosts. It can go through MS ISA Server or MS Proxy doing NTLM authentication. If you're stuck behind a restrictive firewall or proxy with at least one port open for outgoing connections and you have access to a machine outside, you can have nearly full Internet access by using this proxy and the socksify program from dante.
Orion is a window manager for the X11 windowing system. It is written in Scheme and accesses Xlib via the scx library for scsh. It is in fact a family of window managers that can be nested. This enables great flexibility and efficient handling of X11 applications. It currently comes with four child window managers: a conventional moving window manager, a window manager for stacking windows in a frame, a window manager for splitting windows horizontally, and vertically and a work space manager. It also supports saving the window manager layout and programmatic configuration.
QuantLib is a cross-platform, quantitative finance C++ library for modeling, pricing, trading, and risk management in real-life. It is also wrapped as Python/Ruby/Scheme modules. Extensions for Excel, R, and Mathematica are available. Other such extensions are under consideration. QuantLib offers tools that are useful both for practical implementation and for advanced modeling. It features market conventions, yield curve models, solvers, PDEs, Monte Carlo (low-discrepancy included), exotic options, VAR, and so on.
SigScheme is a R5RS Scheme interpreter for embedded use. It features a small footprint (64KB in library form on the "small" configuration) like SIOD and TinyScheme, low memory consumption (2-words per cons cell), handling of multibyte characters (UTF-8, EUCs, and Shift_JIS), and more. It is mainly developed to replace the Scheme interpreter of uim.