Audacity is a free audio editor and recorder. One of the great features of it, is it's compatibility with tons and TONS of file formats. .WAV .MP3 .Ogg, all of these have such devoted following, and audacity supports all of them.
In terms of audio editing, I use it for cutting and modifying audio for a radio station. When working in that context quality is very important. When working on my audio in GarageBand, in the lab, I end up with compressed formats, and consequently bad quality audio.
However when I use audacity, I consistently get good audio samples, and they sound great on radio.
Audacity also has plenty of great effects to help create good sounding audio everytime.
Lastly it has lots of exporting tools, to allow your audio to be converted into a wider range of file formats.
I recommend Audacity to anyone who needs to make quick edits of their audio, and to give it a shot.
Besides, it won the Best Project for Multimedia award at sourceforge.net, and the developer has been employed at Google! (Good for him!)
Lastly remember, it's available for Windows, Mac and Linux!