http://audacity.sourceforge.net/

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. The code is hosted on SourceForge.