Things that contain a liquid
This guess is tempting because 'Aquarium' and 'Drip coffee maker' are defined by the liquids they hold. 'Car engine' also fits, as it contains oil and coolant. However, the theory falls apart on 'Vacuum cleaner' (which deals with air and solids) and 'Email software (targeting spam)', which is entirely digital and contains no physical liquid, breaking the theme on two of the five clues.