SpectralHarp

SpectralHarp lets you strum the sound spectrum in various ways by providing a set of strings that activate different frequencies depending on how they are configured. You can choose the portion of the sound spectrum you want to strum, how many strings to display from that portion, and how to "tune" strings between logarithmically spaced frequencies and linearly spaced frequencies. There are additional knobs for Pitch, Decay, Crush, Spread, and Brightness, which allow you to further modify the sound.

If you want to directly play specific frequencies in the spectrum, you can send MIDI notes to SpectralHarp. The standalone version also supports controlling parameters with MIDI: simply right-click on a control, select MIDI Learn, and then twiddle a knob on your connected device to map it to that control.