GNU Octave: A free and Open-Source numerical computation oriented programming language compatible with MATLAB.
GSL: The General Scientific Library, a free C/C++ numerical library including several mathematical routines, such as random number generators, vector and matrix operations (supporting BLAS) and signal processing methods.
GNU Radio: A free and Open-Source software development toolkit that provides signal processing blocks to implement software radios.
BioSig: An Open-Source software library for biomedical signal processing. It includes both a C/C++ implementation and an Octave/MATLAB toolbox.
FAUST: Functional AUdio STream, a functional programming language specifically designed for real-time signal processing and synthesis.
GDL: A free and Open-Source programming environment compatible with IDL.