Pure Data
graph-based sound language public domain. libpd-- turns PD into a signal processing library.
MaxMSP
graph-based sound language, commercial product
SuperCollider
client-server based sound synthesis language
CSound
well-known synthesis language
The Synthesis ToolKit in C++ (STK)
physical modeling toolkit
athenaCL
modular poly-paradigm algorithmic music composition in a cross-platform interactive command-line environment.
Sig++: Musical Signal Processing in C++
ChucK
Strongly-timed, Concurrent, and On-the-fly Audio Programming Language
The Wekinator
Software for using machine learning to build real-time interactive systems
Puredyne is the USB-bootable GNU/Linux operating system for creative multimedia
liblo is an implementation of the Open Sound Control protocol, for communication among computers, sound synthesizers, and other multimedia devices. Introduction.
Introduction-Computer Audio Overview