DtBlkFx

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.

The last update appears to be in 2008.