SonicBirth (free) began as a student project at École Polytechnique de Montréal by Antoine Missout. It is a tool for creating virtual instrument and effects plug-ins in AU and VST format and has grown steadily since its inception. SonicBirth is similar in spirit to Plogue Bidule, Native Instruments Reaktor, and Cycling '74 Max/MSP. The SonicBirth framework must be installed for the plug-ins to work, but the framework is completely transparent.
At the most rudimentary level, you start with basic modules, hook them together with virtual cables, design a graphical user interface (or not), and hope it all works. Like its brethren, SonicBirth comes with an assortment of prebuilt modules that are interesting in their own right. The best path into this jungle is to first play with these modules — then, if you want to get more involved, modify them or design some simple circuits of your own. At first try, it took me about an hour to build a basic ring modulator with a custom GUI.
SonicBirth user Ulrich Reuter's Buff Rice is among my favorites of the included plug-in effects. It comes without documentation, but it grabs chunks of incoming audio and lets you control various aspects of their playback, including pitch, repeat rate, and phase. Presets like Darth Asthma and Fragmentolettes point the way. The factory plug-in Multiband Scraper is another favorite. It splits incoming audio into two bands, then applies bit crushing and downsampling with different settings for each band. That's great, for example, for adding crunch to the high end of an electric piano while leaving the bass range untouched (see Web Clip 1). Whether you build your own or stick with the growing collection of factory plug-ins, SonicBirth is well worth downloading (http://sonicbirth.sourceforge.net).