A harmonic balancer is a device connected to the crankshaft of an automobile to lessen vibration and ensure that the engine runs smoothly without any unnecessary friction.

When the crankshaft is in motion after a cylinder fires, the power is delivered to the front of the shaft first, this causes a slight twisting motion, sometimes called "torsional twist". This twisting causes vibration, thusly putting extra wear and tear on the engine, shortening its life with every rev.

This is a bad thing.

To counter "torsional twist", a harmonic balancer is put in place at the front of the crankshaft. The balancer is a disc shaped metal ring attached to a piece of rubber. When the shaft twists, the ring and the rubber absorb the vibrations as opposed to the crankshaft and the rest of the engine. This balances the vibrations under the hood, making your car live longer and run more smoothly.

This is a good thing.

Apparently this part can be difficult to replace in some cars. Sometimes the whole engine must be removed from the car itself in order for the balancer to be inserted, or sometimes the radiator need only be removed to get access to it. Removing an engine to replace a part will incur great cost, no matter what garage one goes to.

Harmonic balancer also sounds pretty cool too. It sounds like something you would find aboard the USS Enterprise to regulate shield frequencies or some other crap.

Commander Data: "Captain, they're penetrating our shields with sub space torpedoes. I think we're screwed sir."

Picard: "Nonsense Mr. Data, activate the Harmonic Balancer. You in the yellow jumpsuit, fetch me a cup of tea. Engage"

Thanks to machfive and Kidas for the (sometimes conflicting) info!