SonicBirth

SonicBirth is a very nice toolkit, takes a little time to figure out, be sure to read the manual. Sonic Birth is OPEN SOURCE FREEWARE, not $49, download it and start building cool stuff, or if you are a programmer, please join the Sonic Birth community and start extending the toolkit.

Rich and full featured. I think the only reason Sonic Birth hasn't "taken the world by storm" is that MAX/MSP is the defacto standard for this sort of application. Sonic Birth is easy to use and produces stand alone AU and VST plug-ins, but even those virtues have yet to lure away the folks who are entrenched in the MAX environment.
Even if you're not a "build your own" type, Sonic Birth comes with some pretty good pre-built plug-ins.

Ups...I m on Tiger...

Sadly th imstaller does not wor properly on my system. It doesn't install the app.