Audacity - Audacity is a free and open-source audio production software suite that includes a surprising array of editing tools and recording systems.
Matchering - Open-source audio mastering app that masters music based on any reference track you provide.
TwistedWave - TwistedWave is one of the unique audio editors that allow you to edit your music file online without any installation.
eMastered - eMastered makes a song sound better through online mastering.
Radium - Radium is a music editor with a new type of interface.
Audio Mastering - Pro quality, fully functional audio mastering application for iPad.