Wikipedia
ZynAddSubFX is a free open source software synthesizer for Linux, Mac OS X and Microsoft Windows.
For sound generation it has three hybrid synth engines that combine additive, subtractive, Fourier and other synthesis methods. No external samples are used to produce the sound; everything is done by synthesis. The synthesizer has effects like reverberation, echo, chorus, distortion, equalization and others, and supports microtonal tunings.
The author of ZynAddSubFX is Romanian programmer Nasca Octavian Paul. The project was started in March 2002 and the first public version (1.0.0) was released on September 25, 2002.
A limited Mac OS X version was released by Ben Powers. It has been reported however that this port is PowerPC only (G4-G5 chips).
A Virtual Studio Technology (VST) port implementation is available. The VST version is still in beta development. The latest version features a 'MidiLearn' mechanism (which enables users to assign CC messages to various controls), native VST parameter automation and multiple outputs.