These magnetic register sheets help your furnace or airconditioner use less energy. Use a magnetic vent cover on a ceiling-lever return air register so only lower, cool air is returned to the furnace. Magnetic register covers help save on energy
Easiest home upgrade ever: fancy floor vent covers
Air vent covers, can include 'air flow regulators' which mount behind vent covers. The regulators mount independently of the vent cover and operate with a key, but in some cases you may need to remove the cover.