This board is a simple R-2R DAC paired with an amplifier based on a LM386. The amplifier circuit is based on the 20db gain circuit found in on the LM386's datasheet.
See bom here.
A very basic function generator sketch that uses a timer interupt to generate a triangle wave and port registers to output to the DAC. View here.
This project is under the MIT license. It can be found here.