A voltage controlled high-pass filter based on the AS3320 filter chip. The HPF-3320 can be set to 2 pole or 4 pole mode and it has attenuated CV control over cutoff frequency and resonance.
At high resonance settings the filter will self oscillate. With the cutoff CV attenuated, the filter responds to 1v/octave and as it goes out the shop it is tuned to track over 6 octaves (potentially it can track all 10 octaves and you can tweak the scale on the trimmer marked 'V/OCT' to achieve this - you need a tuner, scope or very good ears).
The HPF-3320's sibling module, the LPF-3320 is also available and the two play very nicely together. Using them in series creates a band-pass with variable Q width, or patching them in parallel you can create a notch-pass, all-pass or band-boost effect. It is interesting to have control over two separate resonance peaks while patching these.