SonicBirth This One is for Mac Users. Free

SonicBirth is an AudioUnit designing application. The first in its category, it allows users to build their own AudioUnit plugins, either by working with existing circuits or by creating circuits completely from scratch. SonicBirth is also a free AudioUnits package, as it comes bundled with a set of plugins designed for standalone use as plugins as well as pre-constructed circuits to modify or build upon for creating new plugins. Whether you're looking for traditional eqs, compressors, and reverbs, or softsynths, distortion, and filters, etc. Every plugin can be used in 32 bits or 64 bits for added precision. Since all plugins share this engine, any improvement in speed will benefit all of them. While Windows users have much choice in modular plugin design environments, sadly Mac user's have relatively few. When I say relatively few, I mean this is the only one that I know off that you can publish a plugin from ... short of moving into C.

Grab it here.