Audacity is a free open source audio editing studio with a no-nonsense interface for Linux, Macintosh, and Windows. It supports mono and stereo tracks for all the basic audio formats and features plug-in support for the more exotic ones. It has a lot of useful features and audio effects from the basic (echo, reverb, fade out, volume adjustments, noise reduction, etc.) to the more complex (tempo change while retaining timbre, vocal elimination, etc.). It can also play both digital audio and MIDI tracks simultaneously.
I found Audacity in the mid-2000s while searching for a replacement for GoldWave, a similar audio studio which sells for $45, and found that nearly all of the features in GoldWave had been duplicated in the free Audacity. I have since stopped using GoldWave altogether and favor Audacity.