miniMO is a low-cost programmable module based on the ATtiny85 8-bit microprocessor. It's available as a kit or assembled. There's a PCB model and a 3d printed version.
Developer Jose Gonzalez had this to say, "I want to encourage people to have a go at modular synths without a fear of breaking the synth, or the bank. For the average price of a typical module you can buy, or build yourself, several miniMOs, which you then load with programs according to your interests. Want to try three LFO's? reprogram three of your modules -no need to buy more dedicated units."
There are a bunch of miniMO videos on YouTube. The video above shows a unit generating sounds with Ring Modulation and Amplitude Modulation; then a second unit is used, working as a low frequency oscillator, to modulate both the Carrier and the Modulator's frequencies.
Pricing and Availability:
See website for options