Audacity

Audacity: Free audio editor and recorder

Audacity is a free digital audio editor and recording application that allows you to record and playback audio as well as export/import WAV, AIFF and MP3 files.

Audacity has many editing functions varying from basic ones such as Cut, Copy and Paste (with unlimited Undo) to mixing tracks together or applying effects to your recorded audio.
The program also has a customizable spectrogram mode, a powerful amplitude envelope editor, a frequency analysis window, full support for VST plug-in effects and numerous built-in effects such as Bass Boost, Wahwah and Noise Removal.