The Pure Data (short Pd) project on SourceForge unifies the extensions (or externals) written for the Pure Data Graphical Computer Music System written by Miller Puckette with contributions from many others.
Pd is certainly powerful and it has massive potential, but it is so extremely frustrating to use. The interface is horrid, the documentation is incomplete, and help is difficult to find. I've wasted massive amounts of time trying to debug problems only to find out that the system itself was screwing up, not my own logic, after closing and reopening my patch. If unseen state can mess up a patch, why is there no reset button? Pd almost seems to wear its spartan design as a badge of honor. I've used Pd for two major projects, but never again. I value my time and effort too much. Looks like it's time to front up and buy Max or learn SuperCollider.