Built a 4-bit synthesizer together with a friend, based on this project. We assembled it on a breadboard, still need to transfer it to a PCB. It is based on an ATMega48 microcontroller, and the sound is generated through a R-2R ladder DAC (the resistors soldered into a chunk in the picture)
Here is a sample of the lo-fi goodness:
The synth is controlled via MIDI, and can generate a single voice in one of 4 waveforms at a time. Several synths can be daisychained to be controlled via one MIDI connector and feed a single output though, so hopefully once we get to soldering everything together we can do that.