Correlometer

There is no to be said about this plugin, it is simple and straitforward, so for me it is adopted. Mainly I will use it to check the mono compatibility of mixes and elements in the mix.

It has also a user manual which is short and concise. They say it can be used to check the phase alignement of vocals and drums which cannot do other free plugin (at least that I know).

I used to use the flux stereotool to check the phase but this one tells you the problematic frequency which can be useful in some cases, so for me it is adopted, it will replace the stereotool.