There's not a lot of choice when it comes to smart smoke detectors. Our favorite for a few years running is the Nest Protect. It's the most full featured alarm, blaring out a loud siren and telling you where the problem is. It works with a lot of other smart home systems, so for example it can turn on all your Philips Hue lights, making it easier to find your way out. However, if you don't want to spend more than $100 on a smoke detector—you should have several in your home—then check out the Roost Smart Battery. This 9 volt Wi Fi connected battery costs just $35, goes into any smoke detector, and alerts your phone when the alarm goes off.