Audacity - Audacity is a free and open-source audio production software suite that includes a surprising array of editing tools and recording systems.
Adobe Audition - Mix, edit, and create audio content in Adobe Audition CC with a comprehensive toolset that includes multitrack, waveform, and spectral display.
TwistedWave - TwistedWave is one of the unique audio editors that allow you to edit your music file online without any installation.
Audio Hijack - Record any audio, with Audio Hijack!
FL Studio - Image-Line's FL Studio, now on it's 12th version, is a well-known music production suite and the most popular beat processor on the market, due no doubt to its longevity. Read more about FL Studio.
WaveShop - Open source audio editor