Electret mic + OpAmp
Takes the output of an Electret Mic, amplifies the signal and passes the output into an Analogue Input or a headphone jack.
This circuit uses an LM385 Dual OpAmp IC to increase the signal strength from an Electret Mic.
All told, it's a very simple amp, and a great starting point for countless hours of tinkering.
Almost all parts on the board come in most Arduino Starter Kits.