This project uses a USB-capable ATmega32u4 to let computers communicate with an arbitrary number of easily-implemented, low-cost MIDI devices. Communication between the 32u4 and other devices uses the I2C bus.
Hardware costs to connect a potentiometer or similar to it are about 1-2 EUR - all you need is an ATmega8 (or anything else that speaks I2C) with two free GPIO pins, a FET, and a resistor. (But I do recommend adding a capacitor between VCC and GND).
At the moment, only control change messages are implemented, but other commands are easy enough to add.