Audacity is free, open source, cross-platform software for recording and editing sounds.
Audacity 2.0 released March 13,2012

http://audacity.sourceforge.net/

from Wikipedia:
Audacity can also be used for post-processing of all types of audio, including podcasts by adding effects such as normalization, trimming, and fading in and out. Audacity has also been used to record and mix entire albums, such as by tUnE-yArDs. It is also currently used in the OCR National Level 2 ICT course for the sound creation unit.

Audacity's features include the following:

  • Importing and exporting of WAV, AIFF, MP3 (via the LAME encoder, downloaded separately), Ogg Vorbis, and all file formats supported by libsndfile library. Versions 1.3.2 and later support Free Lossless Audio Codec (FLAC). Version 1.3.6 and later also support additional formats such as WMA, AAC, AMR and AC3 via the optional FFmpeg library.
  • Recording and playing back sounds
  • Editing via Cut, Copy and Paste (with unlimited levels of Undo)
  • Multitrack mixing
  • A large array of digital effects and plug-ins. Additional effects can be written with Nyquist
  • Built-in LADSPA plug-in support. VST support available through an optional VST Enabler.
  • Amplitude envelope editing
  • Noise removal based on sampling the noise to be removed.
  • Audio spectrum analysis using the Fourier transformation algorithm
  • Support for multi-channel modes with sampling rates up to 96 kHz with 32 bits per sample
  • Precise adjustments to the audio's speed while maintaining pitch (Audacity calls it changing tempo), in order to synchronize it with video or run for a predetermined length of time
  • Changes to the audio's pitch without changing the speed
  • Features of modern multi-track audio software including navigation controls, zoom and single track edit, project pane and XY project navigation, non-destructive and destructive effect processing, audio file manipulation (cut, copy, paste)
  • Conversion of cassette tapes or records into digital tracks by automatically splitting the audio source into multiple tracks based on silences in the source material
  • Support for multiple platforms Audacity works on Windows, Mac OS X, and Unix-like systems (including Linux and BSD), among others
    • The latest stable version supports Windows 98/ME/2000/XP/Vista/7, but Windows 95 and NT are not supported.
  • Audacity uses the wxWidgets software library to provide a similar graphical user interface on several different operating systems.