There are a lot of great stereo imaging tools and spectrum analyzers. Most of them aren't free though. Having a good way to visually analyze loudness, frequency distribution and stereo image is incredibly important. Unfortunately, it is often the case that most default or free solutions are barely functional. They don't provide you with enough information to shape your mix or don't present it in a simple, clear and understandable format. (I'm looking at you, FL studio visualizers!)
SPAN is a spectrum analysis tool that will let you monitor your peak volume, RMS and LUFS. It shows you the frequencies that are passing through the plugin and you can customize the spectrum view by changing time, frequency and level ranges. If you want even more precision and control, you can adjust the fast fourier transform sample size, it gets that technical! You can also compare channels, left and right of the same channel or two different ones. Two channel comparison is the limit of the free version, SPAN Plus allows you to display as many channels as your DAW will allow. It also features a correlation meter that measures your phase alignment (1=fully aligned and -1=completely misaligned). It can also track your loudness stats thoughout the playtime to iron out any kinks. Handy, functional and no-nonsense tool.
MSED is a stereo analysis and encoding tool. It can help you manange your side and mid channels: your stereo image. It comes with a basic set of tools that allows you to pan, change levels, swap left-right channels and flip phase 180 degrees. The visualizers are pretty simple, you have your correlation meter from before, stereo pan meter and plasma-style vector scope. You might think you don't need it if you DAW provides stereo imaging functions of its own, but often times you're going to be playing guessing games in terms of how that audio will actually get processed: some DAWS merge your stereo tracks to mono, some don't. MSED takes care of that ambiguity and puts all of the necessary tools and monitoring in one spot.
These plugins are available in VST, VST3, AU and AAX for free! I really can't recommend them enough if you're starting out producing or are in need of simple tools that won't take a toll on your CPU with fancy advanced processing.
SPAN Product page: https://www.voxengo.com/product/span/
MSED Product page: https://www.voxengo.com/product/msed/
