Matchering - Open-source audio mastering app that masters music based on any reference track you provide.
Jokosher - Jokosher is a simple and powerful multi-track studio.
Audio Mastering - Pro quality, fully functional audio mastering application for iPad.
Fission - Edit audio in minutes, not hours.
eMastered - eMastered makes a song sound better through online mastering.
Audacity - Audacity is a free and open-source audio production software suite that includes a surprising array of editing tools and recording systems.