I’ve recently installed MOTU Audio Tools which is part of older Ultralite/AVB models driver, it does not come as standalone app installation only part of the driver, it works flawlessly with my Ultralite mk5 which has it’s own driver installed, my guess is that it would work for any audio interface as you can select the channels. it has oscilloscope, freq analyzer, phase and stereo image built in, I really like it except for the fact that you can’t stack screens, you have to switch between them when you want different view. other then that great tool.
it’s part of the driver for Ultralite mk4/AVB units, you can’t install it standalone, but the parallel drivers runs on my mac studio without any issues. after installing the driver the MOTU Audio Tools app is available as a regular app, it runs all the time and connected to loopback, which makes it easy for me to take a look at a spectrum of something I play from youtube for example, or when I just play my synths standalone…
btw OP, if you want a VST version that would run in a daw for example, this one is free and great:
the great thing about that one is that you can sidechain secondary signal to it resulting in two scope structures with clear image of how the sounds interact, very useful for visualizing bass ducking from a kick for example
not exactly a plugin but there’s a JS Oscilloscope in Reaper. I also used an AU (maybe there’s a VST version too, not sure about that) called s(M)exoscope, it’s a bit dated though but works.
Another one you could have a look at is Signalizer which comes with some extra features as well
Bitwig has an oscilloscope plugin too (“Scope”), I use it a lot for tuning. melda Production have an oscilloscope in their free bundle. I’ve never used it but I’ve used other Melda plugins and found them pretty good.
Blue Cat Audio has several audio analysis tools including an oscilloscope. It is not cheap compared to some of the other options in this thread, but it does allow one to compare multiple signals, take precise measurements, and do other “audio engineer” stuff.