Used to be, your doorbell could only let you know when someone was at the door. Functional and efficient, but limited. Nowadays, via modern technology, your doorbell will not just tell you that someone is there, but you can see them and take their picture (or video). Some doorbells can even tell you who is there — and you don't even need to be at home. These four smart doorbells are CNET's picks for the top of the line.