The most popular smart device for you home is the Smart Phone, with it you can have linked to it your Home security system, alarm, cameras, the boom box, fridge, solar system, just about anything.
The next most popular smart device is the home music or sound system, like Sonos, Bose or google nest.
Coming in next is Smart Front Door Bells, that will alert on your smart phone, allow you to open or just talk with whoever is at the front door. Even recording who rang the door and any messages left.
Then there are a whole range of different devices out there, take care though each is likely to have their own apps and method of communicating that may not be compatible with your other systems.