Blue Cat's AcouFiend is an acoustic feedback simulator plug-in that can make your guitars scream, growl, or sing, even at low volume or through headphones, and it gives you total control over it.
AcouFiend simulates the acoustic feedback (a.k.a "Larsen effect") well known to guitarists playing loud and close to their speaker... Except that here, you have total control over the effect.
You can choose when and how to trigger the feedback, on sustained notes or chords. The plug-in lets you select the main harmonic of the feedback, and you can even choose to transpose it for more creative effects.
Of course, it can be entirely automated or controlled via MIDI.
And since it is a plug-in, it is not just for guitars: create screaming synths, sustained vocal harmonies and more..