SPAN by Voxengo is the free spectrum analyzer that mastering engineers actually recommend. It shows you the frequency content of your audio in real time, with configurable FFT settings, stereo/mono/mid-side analysis, and EBU R128 metering. The feature I use most is Ctrl+drag to solo a frequency range while listening. It’s a fast way to find problem frequencies in a mix. 1. The interface won’t win any design awards, but it’s functional and super customizable.
I keep SPAN on my master bus in every project. Having a visual reference for your frequency balance makes you a better mixer, period.