There are thousands of VST plugins that concentrate on really traditional functions like compressors, reverbs, limiters. There are much less tools that take different approaches to sound editing. 
This one, DTBFX, is such a one. I already got to know it in 2007 but hadn’t really used it very much until yesterday when I wanted to do something entirely different. 
So what is it? It is a tool that filters, smears, pitches up and/or down but it enables you to do so on a specific part of the frequency spectrum. Whereas most of the tools have an effect on all or most frequencies, with DTBFX you can limit the bandwidth rather precise. With surprising results.
In his own words:
DtBlkFx by Darrell Barrell is a freeware Fast-Fourier-Transform(FFT) based multi effect VST plug-in for Windows and Mac.

  • Precision parametric equalizing with sharp-roll off, adjust individual harmonics of a sound.
  • Harmonic based (or comb) filtering, including active harmonic tracking.
  • Various types of noise control, change contrast between loud and soft frequencies, clip frequencies or apply sound smearing.
  • Frequency shifting, harmonic and non-harmonic shifting, including active harmonic repitch.
  • Various methods of mixing left and right channels, standard and harmonic vocoding, convolution like mixing and more.
  • Frequency masking, set harmonic or threshold masks for any effect.
  • Up to 8 effects in series.
  • GUI graphics can be customized.
DtBlkFx is available for free as a VST effect plug-in for Windows and Mac (last update: 15 Apr, 2008, Windows version).

Go get DTBFX [ here ]