MAnalyzer

This spectrum analyzer offers in comparison to SPAN advanced visual help for detecting frequencies and seeing their relation to octaves, keyboard notes and certain mix regions (as part of the "Areas" section).

Huge downside is that you don't get a big enough view - the spectrogram is way too small (depite you can zoom in) due to Melda's policy that all of their free plugins aren't resizable. You need to upgrade for 49$ to get this feature.

Hence that, I think for most use cases I will continue to use SPAN but for frequency detection and assignment to the relative keys, I will come back to use this tool.