oscilloscope
It has an useful mode for viewing test tones (constant frequency), called Envelope, which stabilizes the waveform so it won’t slide right to left anymore, making easier to compare signals or observe how tiny amounts of wave-shaping are affecting the test signal.
🔴 pressplay-music.com/wave-observer-pro
Press Play Wave Observer Pro
complete metering suite
Stereo VU-meters, Mid/Side meters, spectrum analyzer, spectrogram, loudness and correlation. I find the spectrogram mandatory when it comes to double-checking the low end. It can clearly display the shape of the kick and each bass harmonic. In this spectrogram (FFT Size setting: 2048, FFT Overlap: 16x) you can clearly see the bass notes (at the very bottom, those long darker reds with lighter red fades) and the kick drum (right in the middle of the graph there are two isolated kicks, you can see them clearly as the long bass notes are missing).
🔴 ikmultimedia.com/products/trmetering
IK Multimedia Metering. This is Metering version 5, the newest version is 6, but this older one displays better (no spectrogram artifacts) on my Windows PC.
metering with a twist
This one is hard to master. Not even the iZotope tutors are using it right. I’ll probably write a dedicated resource on the topic. But until then, first and foremost, don’t try to conform your song to this guide by only using EQ. More often than not, dynamics control is what is needed. Always read its measurement as a function of tonal balance and dynamics (transients level).
iZotope Tonal Balance Control
probably the most feature-rich A/B tool out there
In terms of metering, this one doesn’t do anything more than the (free) tools above. However, it can make referencing easier. You can map keyboard keys (or a MIDI controller) and remote control the A/B switch (your audio/reference) and also map the track selector (it can load up to 16 references), or any other functions you may need. It really makes the referencing process easier.