CLASSIC FREE SOFTWARE
Jim Bumgardner’s syd
Released two decades ago, Syd is a crossplatform modular synthesis environment. It shares some similarities with other graphical synthesis programs such as Reaktor, SynthEdit, and Max. However, Syd is a strictly offline affair – sounds are made by patching up various modules with virtual patch cables, after which they can be played back.
Along with basics like oscillators, mixers, and envelopes, there are Butterworth and 2nd order filters, Karplus/Strong plucked string generators, and the ability to play back CSound scores. Still available (as a Java app), Syd remains a great way to while away the hours. https://jbum.com