Audacity - Enjoy Audio Mixing

With so many audio editors out there, finding the right tool for the job can be tricky. The multiplatform, open-source editor Audacity has leaped to the top of our list with its clean interface, excellent features, and support for 32-bit floating-point audio. Audacity succeeds at being both feature-rich and flexible, while keeping the toolbar and work space uncluttered. It does this by relegating most functions--such as effects, edits, and project management--to handy drop-down menus.
Audacity supports uncompressed audio standards such as WAV and AIFF in addition to OGG and MP3 files. A full complement of basic effects such as reverb, delay, and compression are included. Virtual Studio Technology (VST), Linux Audio Developers Simple Plugin Architecture (LAPSDA), and Nyquist plug-ins also are available if you're looking to expand. Another eye-catching feature is the Beat Analyzer, which helps you peg the beats per minute of a phrase.
The performance of most functions is quick--even CPU-hogging tasks such as exporting to MP3 or adding complex effects. Though Audacity doesn't offer advanced effects or features out of the box, its usability and price set this freeware wonder above the crowd.

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From Audacity Development Team : 

This editor can record and play sounds and import and export WAV, AIFF, MP3, and OGG files. Edit your sounds using cut, copy, and paste features (with unlimited undo functionality), mix tracks, or apply effects to your recordings. The program 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, wah wah, and noise removal, and the program also supports VST plug-in effects.
Version 1.2.6 enables FLAC support and contains some bug fixes.

What's new in this version: Version 2.0 improves many effects, improves track labeling, features automatic crash recovery, and more