Audacity is an audio editing software that is free, open-source and runs on Mac OS, Linux & Windows operating systems. It is a great tool for audio enthusiasts, who wish to get into the basics of audio editing and processing, without spending a dime.
It offers Equalization, compression, fades, tempos, and other basic tools to get you started. The GUI looks childish, however its performance is not. Audacity also supports addition plugins and libraries.