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.
TwistedWave is a very easy-to-use and powerful audio editor. It supports Audio Unit plug-ins and many file formats, including wav, aiff, caf, mp3, mp4, FLAC, and Ogg Vorbis. It has good multichannel support, so you can apply Audio Units to a selection of channels. It supports unlimited undo and redo, which work instantly. Zooming in and moving around the waveform is very fast, even with very large files. All the effects are applied in the background, saving a lot of time.
eINIT is an alternative init system. It's quite fast, since it can potentially be used without the help of any scripting at all, but it is still flexible and extremely modular. The focus is on speed and parallelisation, mostly with embedded devices and low-downtime servers in mind, and benchmarks do suggest that it's doing a fairly good job at that. It compiles cleanly (and should thus work, provided someone writes appropriate modules) on Linux, FreeBSD, and Darwin/Mac OS X.
HopSlide is a tool for authoring and playing slides. It supports animations, annotations (slide overlays), math formulas (MathML support and TeX syntax support), SVG images, and canvas graphical effects. HopSlide can be used to compile slides into XHTML files. These files can be used by XHTML capable browsers (e.g., Firefox). Compiled slide files can be used to play the slides without HopSlide.
Halyard is a scriptable multimedia engine with support for video, audio, graphics, and user interaction. It currently runs on Windows, and has a preliminary Macintosh port. It is based on PLT Scheme, wxWidgets, and the Quake 2 Engine. It includes a rudimentary IDE based on Scintilla.
scheme-asteroid is a real-time asteroid-like game. It has a neat object-oriented architecture, includes a real-time physics engine, and utilizes a triple-buffering display system. All graphics and artwork (e.g. background images) are generated on-the-fly using fractals and recursive procedures.
mu is a collection of utilities for maildirs, in particular for indexing and searching them. mu fills a database with information about your emails, which can then be queried using an easy query language. It can work stand-alone or be integrated with a mail client. It also provides an emacs-based email client ("mu4e") and bindings for Guile/Scheme.