Matchering - Open-source audio mastering app that masters music based on any reference track you provide.
iZotope Ozone - Master audio with Ozone 9, the industry’s most comprehensive mastering software. Use intelligent DSP, spectral shaping, Tonal Balance Control, Low End Focus, Master Rebalance, and more.
eMastered - eMastered makes a song sound better through online mastering.
Fabfilter Pro C - High-quality compressor plug-in for the most demanding engineers
Audio Mastering - Pro quality, fully functional audio mastering application for iPad.
Maximus - Image-Line's advanced compressor, limiter and maximizer plugin.