Sound Distorter is a chiptune synth, based on the Atmega168. It runs two threads - a generator thread that computes stereo samples at 32 kHz and a sequencer thread that processes the song events at 100 Hz. The synth has a basic menu-based UI in the form of a display, a slider and buttons. Schematics, code and detailed explanation are available here.
Same HW, running as a monophonic MIDI synthesizer: here.