Hack-a-day has published another synth DIY project - a tutorial on building a basic digital synthesizer:

This simple guide will show you how to build a digital synthesizer that generates and manipulates square waves. Your synthesizer will have one oscillator, which produces a variable pitch controlled by a potentiometer, as well as an LFO which modulates that pitch at a variable frequency. The part count for this project is quite low, and it can be built for under $20.

Looks like a fun project to learn about basic digital sound generation.