SQUASCILLATOR: Square-wave AVR VCO - PCB/panel + firmware (4HP)
3 in stock.
A square-wave oscillator with voltage-controlled duty cycle - this one is built around a microcontroller, rather than an analogue, core so far fewer components are required to get things up and running and it's far less hassle to calibrate; there's a fine-tune trimmer on the board but most of the heavy-lifting is done by the firmware.
Like my other AVR-based VCOs, this one will track 1V/oct pretty well over 5 octaves, although if you're expecting 'to-the-cent' accuracy then you may well be disappointed unless you're prepared to hack on the firmware (specifically, fine-tune the values in the pitch lookup table - have fun!)
Apropos of which, the firmware is available for modification and has a permissive license so feel free to hack on it.
Square waves aren't as boring as they seem - with a bit of judicious filtering (resonant or otherwise) you can get some pretty crazy sounds from your bog-standard square, and team a couple of these up with something like the CLOCK-LOGIC module and you've got a recipe for wild and crazy sounds.
This is a PCB/panel set - the firmware IC is but it is up to you to provide the necessary bits to complete the build. There are a couple of surface-mount components which have been pre-soldered.
The schematic, BOM and firmware code can be found here: https://github.com/yorkmodular/squascillator-vco