IMPORTANT: By buying this item I'm going to assume that you have enough knowledge to hold a soldering iron at the right end and read a schematic.
Analogue VCO cores are a pain in the ass to build so this is based around a microcontroller instead - cheating? Maybe so, but it's pretty damned versatile and if you have the knowledge to program microcontrollers then you can hack it to your heart's content. What makes things a bit more interesting is that both the output frequency and the waveform can be voltage controlled - normally, the waveform would be selected via a pot alone, but I decided it would be interesting to allow voltage controlled waveform selection too. Just because.
As supplied, the module can provide six different output waveforms: sine, triangle, square, sawtooth, something pulse-ish and digital noise - waveform selection is either via the pot _or_ it can be voltage controlled. If you want to get down and dirty with the code then you could easily replace the wavetables with something else. It'll track 1V/octave pretty well over 5 octaves, this being governed by the fact that microcontrollers tend to get a bit upset if you feed more than 5V to their input pins.
There are three inputs (CV, Wave and OP) - the OP input can be used to do interesting things, although by default it'll allow you to alter the duty cycle of the square-wave output and introduce some nasty-sounding distortion into everything else.
In addition, these modules were designed to fit behind 2HP panels however in some cases there's a risk of interference with adjacent modules particularly if these modules have PCBs which are the full width of their panel, so they're supplied with a 3HP panel as standard.
The schematic, source and BOM for the module can be found here, as can panel files: https://bitbucket.org/yorkmodular/avr-vco-diy/src/master/ - a pre-programmed firmware chip is included .The firmware source has a permissive license, so feel free to hack on it - updates and modifications are most welcome
Once assembled, the module is 47mm deep from the rear face of the panel to the rear edge of the power connector. Please bear this in mind if you're using a shallow skiff system.