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.
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.
eMastered - eMastered makes a song sound better through online mastering.
LMMS - Make music with a free, cross-platform tool
Audio Mastering - Pro quality, fully functional audio mastering application for iPad.