Pure Data was developed by Miller Puckette as a fully open source version of the work that he originally did for Max while at IRCAM in the late 80’s and early 90’s. It is lesser known to some extent by musicians but definitely worth checking out. Max, now developed by Cycling ‘74, draws heavily from Pd.

from wikipedia:

“Pd is very similar in scope and design to Puckette’s original Max program (developed while he was at IRCAM), and is to some degree interoperable with Max/MSP, the commercial successor to the Max language. Both Pd and Max are arguably examples of Dataflow programming languages. In such languages, functions or “objects” are linked or “patched” together in a graphical environment which models the flow of the control and audio. Unlike the original version of Max, however, Pd was always designed to do control-rate and audio processing on the host CPU, rather than offloading the synthesis and signal processing to a DSP board (such as the Ariel ISPW which was used for Max/FTS). Pd code forms the basis of David Zicarelli’s MSP extensions to the Max language to do software audio processing.”