Otemrellik made a nifty polyphonic synthesizer with a custom 3D printed case.

This portable, compact, synth includes a portable speaker and can run off 2 AA batteries or use USB for power.  It features 8 note polyphony with each note having 2 voices.  This little synth is packed with features.   Oh, and the LEDs on the buttons respond to the speed and shape of the low frequency oscillator (LFO)

Code for the project can be found on GitHub

Plans for the 3D printed case can be found on Thingverse