Audacity - Audacity is a free and open-source audio production software suite that includes a surprising array of editing tools and recording systems.
Super Sound - Super Sound is a resourceful audio and music editor.
TwistedWave - TwistedWave is one of the unique audio editors that allow you to edit your music file online without any installation.
Add Audio to Video - Add Audio to Video empowers you to add custom audio tracks to your video files.
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.
MixPad - MixPad is free music Music Mixer and Recording software designed for easy audio production.