DtBlkFx is a superb spectral processor, and it's free! It has about 30 different algorithms. You can process as many as 8 algorithms in a series, but usually 1 algorithm is enough.
It reminds me of Native Instrument's discontinued Spectral Delay.
The UI shows a nice sonogram of both the original input and DtBlkFx's output. There is a Delay between the input and the output, this value defaults to 1 beat, but you can reduce that so it's almost instantaneous.
The documentation appears well done. I just goofed with parameters until I figured it out.
The UI is somewhat unusual regarding how to adjust coarse and fine values, but it's actually convenient when you get used to it.