JeremyCumbo

SonicBirth is a plugin development environment - free, but currently unmaintained. Nonetheless, it provides some powerful functionality if you want to get into the details of tweaking your sounds. It can generate AudioUnit and VST plugins (others?) but AudioUnit plugins are what it was designed for. It consists of a standalone application where, in a Max/MSP-like gui, you piece together audio processing pieces. Then, when satisfied with what you’ve created, you can design a gui and export as a plugin (a ‘.component’ file for AU plugins).

SonicBirth provides a long list of components to use in creating plugins. The plugins produced are certainly not perfect - you may have to modify the plugin a few times to get it the way you want - but in general very usable.