For the first part of my modular synth exploration, I wanted a cheap and simple VCO (Voltage Controlled Oscillator) that used only parts I had on hand.  After some searching I found this design: It only uses two op amps and some other parts I had on hand.  I threw it together on the breadboard pretty quick and it seems like a decent start towards my goal of making the cheapest and most simple modular synth I can.  Listen to a demo below.