PC users have had various simple ways of constructing custom VST plugins (including the popular SynthEdit), but now Mac users can get in on the action:

is an environment for creating custom Audio Units for Mac OS X, via a
modular 'patch cord'-style interface. The interface is very
approachable for this kind of system, and MIDI control is easy to
implement. (Note the more-than-passing resemblance to Ableton Live's
knobs.) It comes with a number of included prebuilt plugins, with more
planned for the future.

Now the bad news: you can share your AUs, but only to owners of the
program. It would have been nice to see a free runtime. And this isn't
necessarily radically different from deeper environments like Reaktor
and Max/MSP, though the simplicity is part of the appeal (and has been
for the PC options).

Anyone checking this out, let me know.

Cost: US$99 preview; US$129 when released — graphic designers, build a beautiful UI and get it free
Availability: Final release June 2005