Engine Immobilisers
Sometimes referred to as 'electrical immobilisers', these will disable the engine from starting when activated by illegal entry into your car and are generally designed to cut out the power supply to the engine starter motor.
Immobilisers are highly effective against having your car stolen. They require a little bit of competence to fit but are a hassle free solution once they're up and running. They are generally automatically armed and disarmed by the presence of a 'magic bullet' or transmitter which you attach to your car keys.