Quoted from: http://audacity.sourceforge.net/

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Audacity is a free audio editor. You can record sounds, play sounds, import and export WAV, AIFF, Ogg Vorbis, and MP3 files, and more. Use it to edit your sounds using Cut, Copy and Paste (with unlimited Undo), mix tracks together, or apply effects to your recordings. It also has a built-in amplitude envelope editor, a customizable spectrogram mode and a frequency analysis window for audio analysis applications. Built-in effects include Echo, Change Tempo, and Noise Removal, and it also supports VST and LADSPA plug-in effects.

Audacity is being developed by a team of volunteers under the open-source model. It is written in C and C++, using the wxWidgets cross-platform toolkit. All of the source code to the program is made available under the GNU General Public License, which essentially allows anyone to modify the source code as long as they publicize the changes.

Audacity 1.2 supports LADSPA and Nyquist plug-ins natively, and VST plug-ins with the optional Audacity VST Enabler.

Audacity 1.2.0 already comes with some sample LADSPA and Nyquist plug-ins in the Plug-ins folder. Put the VST Enabler and more plug-ins in this folder in order to use them with Audacity. The next time you open Audacity, the Plug-ins will be in your Effect menu (or possible Generate or Analyze). Plug-in effects always appear at the bottom of these menus, with built-in effects at the top.

Note: most Mac VST plug-ins do not work on Mac OS X. You need special Carbon VST plug-ins.