The August Smart Lock unshackles your home from the tyranny of physical keys. The 3-inch cylinder fastens to the back of an existing deadbolt, leaving the outside appearance of your door unchanged. Once attached, a Bluetooth signal from your smartphone can flip the mechanism to unlock the door. A guest with a smartphone and an August app can enter once you've granted them access. No more passing out hard-copy keys, which are much harder to revoke—and easier to lose. The Smart Lock lets you pick days and times for guest entry, so the dog walker can get in only during lunch hours, or your weekend Airbnb guests lose access once their stay is over. If your guests don't have smartphones, a Wi-Fi add-on allows you to buzz them in remotely. Plus, you can set the Smart Lock to bolt automatically every time the door shuts. And should anything fail, the old key still works. $229, plus $50 for remote connectivity Image: Jonathon Kambouris Share Win the Holidays with PopSci's Gift Guides Shopping for, well, anyone? The PopSci team’s holiday gift recommendations mean you’ll never need to buy another last-minute gift card. SEE GIFT GUIDES