Audacity

Audacity is a free, cross-platform audio editor written in C and C++ using the wxWidgets toolkit. It was started in 1999.

Advantages:
 * Open source (Released under the GPL License)
 * Cross-platform - (Linux, Mac, and Windows)
 * Multiple tracks
 * Capable of Ogg, FLAC and MP3 encoding/decoding, the latter with use of an external lame-encoder.

Disadvantages:
 * MP3 encoding is not included (the algorithm is patented), but you can still download it for free.

= External links =
 * Audacity sourceforge page
 * Audacity screenshots
 * Audacity Wiki
 * Audacity user forum