Audacity – FREE – A free audio editor. You can record sounds, play sounds, import and export WAV, AIFF, 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 Bass Boost, Wahwah, and Noise Removal.
GoldWave – A very flexible digital audio editor. Features include: play, edit, mix, and analyze audio, apply special effects such as fade, equalizer, Doppler, mechanize, echo, reverse, flanger, and more noise reduction and pop/click filters. Make perfect digital copies of audio CD tracks using the CD Reader tool and save them in MP3, WMA, or Vorbis files View a variety of real-time visuals and VU meters Convert files to/from different formats, such as wav, mp3, ogg, iff, au, vox and even raw binary data. Not free, but there is a demo version available.
Jasmine Audio – An audio editor, mixer, and file converter for Macintosh. Convert multiple audio file types including MP3, WAV, AIFF, QuickTime, MP4 (AAC), AVI, LAW, System 7, 3G. Change pitch/tempo: Speed up or slow down your music without changing the pitch. Or modify the pitch without changing the tempo. Overdub recording: Record a new track while one or more tracks play. MultiLevel Undo/Redo, and MultiTrack capabilities.