Sonic Pi - Sonic Pi is a new kind of instrument for a new generation of musicians. It is simple to learn, powerful enough for live performances and free to download.
VCV Rack - A cross-platform modular synthesizer.
SuperCollider - A real time audio synthesis engine, and an object-oriented programming language specialised for...
miRack - Eurorack-style modular synthesizer for iOS.
ChucK - A strongly-timed music programming language
SynthEdit - Visual programming software to build your own synthesizer.