The Squarecave is a cool synth DIY project by Logan Williams, a high school student from Newport, Oregon.
The Squarecave is a hybrid digital and analog music synthesizer. It uses digital CMOS ICs to generate a sequence of oscillations of varying pitch and volume, and then pipes the result through analog filters (lo-pass, hi-pass, band-pass).
Visit his very nice site for details, schematics and more.
You can preview the sound of the Squarecave below.