SonicBirth, a Mac OS X app for for constructing VST and AU plug-in instruments and sound processors, is now free and open source:

SonicBirth is now freeware, and open source (GPL). We are looking for developer to further work on and enhance SonicBirth. Work for the major version of SonicBirth is starting. This next version will be a complete rewrite and a commercial product.

Based on the developer’s note, it sounds like there may be a new commercial version of the product if the open source version gains any traction.

SonicBirth works within any AU host and lets you edit and create VST/AU instruments and effects. A variety of sample instruments and effects are included.