A bare piezoelectric transducer that can be used either as a sensor, or as a speaker.
Good to have a few of these around.
A handy component for all kinds of projects, as they can both produce sound and revive it. Not that all speakers and mics cant, but as far as cheep components go they are rather versatile.
MCM is a great company. Very fast service and shipping of Raspberry Pi. Will tell all my friends to buy from MCM