Sonifyer
http://www.sonifyer.org
Sonifyer is a stand-alone application for data sonification and visualization. Sonifyer incorporates two sonification methods: 1. audification or 2. parameter mapping, based on FM synthesis.

Csounds
http://www.csounds.com/
Csound's history lies deep in the roots of computer music, however, as it is a direct descendant of the oldest computer-program for sound synthesis, 'MusicN' by Max Mathews. Csound is free, distributed under the LPGL licence and it is tended and expanded by a core of developers with support from a wider community.

SuperCollider
http://supercollider.sourceforge.net/
SuperCollider is an environment and programming language for real time audio synthesis and algorithmic composition. It provides an interpreted object-oriented language which functions as a network client to a state of the art, realtime sound synthesis server. SuperCollider is GPL licensed and by musicians, scientists, and artists working with sound.

HighC
http://highc.org/
HighC is a graphical music creation tool. It is a synthesizer, a sequencer and a mixer inspired by Iannis Xenakis' UPIC work of the mid 80's. HighC uses a structured (additive) synthesis model allowing composition of elementary sound objects into higher level elements. Roughly said, HighC is to HyperUpic and its descendants what PowerPoint is to Photoshop.

IanniX
http://www.iannix.org/
IanniX is a graphical open-source sequencer, based on Iannis Xenakis works, for digital art. IanniX syncs via Open Sound Control (OSC) events and curves to your real-time environment. IanniX is free and open source and is licensed under GPL3.

MetaSynth 5
http://www.uisoftware.com/MetaSynth/index.php
MetaSynth 5 is an application that allows the creation of sound from images. It was most notably used on The Matrix for bullet-speed special effects, and also appeared in Aphex Twin's song entitled 'Equation'.

Audacity
http://audacity.sourceforge.net/
Audacity is a free, easy-to-use and multilingual audio editor and recorder for Windows, Mac OS X, GNU/Linux and other operating systems. Audacity can also be used for post-processing adding effects such as normalization, trimming, and fading in and out, though lacks dynamic equalizer controls, real time effects and support for scrubbing and MIDI files can only be displayed.

Ableton Live
http://www.ableton.com/live
Ableton Live is a loop-based software music sequencer and Digital audio workstation. Live is designed to be an instrument for live performances as well as a tool for composing and arranging. It used for mixing of tracks by DJs, offering a suite of controls for beatmatching, crossfading, and other effects.

Comparison of Audio Synthesis Environments
http://en.wikipedia.org/wiki/Comparison_of_Audio_Synthesis_Environments