gFractor

I came across this plugin while searching for a goniometer, and what I got exceeded all my expectations. The best part of this plugin is the spectrum analyzer, divided into seven sections from bass to treble. Clicking on a section only allows you to hear the sound from that section, while the remaining harmonics are muted. This option is a lifesaver for mixing and mastering, and it works and sounds excellent. It also includes an audio visualization spectrogram, a LUFS meter, and a host of other useful features. It's also well-optimized. It's a great and very useful plugin.