Audacity

Audacity is a free audio recorder/editor/mixer. 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, and it also supports VST plug-in effects.

Note: While the software is classified as free, it is actually donationware. Please consider making a donation to help support development.

• Adds previously missing support for Magic Mouse/Trackpad (thanks to Paul Licameli).
• New Distortion effect, Rhythm Track (was called Click Track) and Sample Data Import (thanks to Steve Daulton). Also effects are no longer grayed-out when in pause.
• New scrub ruler and ‘pinned’ option so the waveform moves and the recording/playing head stays still. (Paul)
• New features in Timer Record (Mark Young & Steve)
• On Mac, audacity.app is now added to /Applications/ directly rather than adding a folder there.
• 64 bugs/annoyances in 2.1.2 fixed of which 15 were long-standing issues. Also lots done (Paul again) to systematically deal with memory leaks.