Audacity is a free audio editor. With Audacity, you can record sounds, play sounds, import and export WAV, AIFF, Ogg Vorbis, 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 Echo, Change Tempo, and Noise Removal, and it also supports VST and LADSPA plug-in effects.
Pros: Audacity is a highly valuable application (free). It supports a variety of formats and its learning curve is not as steep as most commercial audio editors. Audacity is a cross-platform application for Mac, Windows, and Linux.
Cons: Audacity can be somewhat unstable at times, but free is free (remember to save often).