Audacity – Best Free Audio Editor, Recorder

Audacity is free, open source cross-platform software for recording and editing sounds.
Some of the important features:

• Record from microphone, line input, or other sources.
• Dub over existing tracks to create multi-track recordings.
• Import and export WAV, AIFF, AU, and Ogg Vorbis files
• Import and export all file formats supported by libsndfile
• Edit and mix an unlimited number of tracks.
• Alter frequencies with Equalization, FFT Filter, and Bass Boost effects
• Record and edit 16-bit, 24-bit, and 32-bit (floating point) samples
• Runs on Mac OS X, Windows, and GNU/Linux

Download latest version on Audacity:
http://audacity.sourceforge.net/download/