The pedalSHIELD Arduino guitar pedal by Electrosmash:

The shield has three parts:

• The Input Stage or Preamp: Amplifies the guitar input signal and sends it to the Arduino microcontroller to be processed.
• Arduino Board: It does all the Digital Signal Processing (DSP) modifying the signal and adding the effect (delay, echo, distortion, volume…).
• The Output Stage: Once the waveform is processed, the signal is taken from the Arduino DACs and prepared to be sent to the Guitar Amplifier. This part also includes a Summing Amplifier which is very useful for delay effects like echo or chorus.

Via Hackedgadgets.