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.
Reaper - Reaper is a focused digital audio workstation (DAW) developed by Cockos. In the creation of the software, the digital audio technology company intended to make audio editing accessible to the masses.
eMastered - eMastered makes a song sound better through online mastering.
GarageBand - GarageBand is a fully equipped music creation studio for Mac
Audio Mastering - Pro quality, fully functional audio mastering application for iPad.