I'll admit that most of the apps on my smartphone are fairly useless. For every productivity program on my iPhone, I've got two or three games intended for 8-year-old kids.
However, I do have some really awesome apps that control various systems in my home. Smartphones can essentially control any electronic device enabled for Bluetooth or Wi-Fi, and with a few quick home improvements, I've made my home much more smartphone friendly.
With the right app, a smartphone can control:
HVAC systems -- You don't need to replace your entire HVAC system to enjoy full smartphone control. After all, think about how you currently control your HVAC system. You've probably got a digital control on your wall, and by replacing it with a Wi-Fi capable control, you can monitor your home temperature through any smartphone's Web browser. Some thermostats even have dedicated apps.
Why would you want to control your thermostat from your phone? Well, for starters, you can turn on the air conditioner while you're at work if a heat wave suddenly rolls through town. Likewise, you can turn the system off if you're going to be away from your house for longer than expected. It's an easy way to save money and stay comfortable.
Stereo systems -- Ideally, you could ditch every remote in your house and just use your smartphone to control all of your devices, but you can't really do this without extra hardware -- and even then, smartphone remote control apps are buggy and don't work with all of your devices.
Still, you can generally control newer stereos with your smartphone by turning on Bluetooth in the settings menu. When you buy a stereo, look for a Bluetooth-capable receiver. There are also a few adapters out there that allow certain smartphones to work with specific stereos, but be sure to check the reviews before you order; some don't work especially well, and there's nothing as frustrating as a bad remote.
Home security systems -- Worrying about your home isn't paranoia; over 2 million home burglaries occur in the United States each year. You can enjoy peace of mind while you're away from home by investing in a quality security system, and many modern providers advertise "smartphone security" services that let you check up on your home virtually anywhere.
Depending on how involved you want to get, you can opt for a simple Wi-Fi-equipped nanny cam or a full-fledged system that allows you to turn your security system on and off remotely. You can install a few cameras and some security software yourself for fast security, but the more complex systems require a professional installation.
Before buying a security system or any other smartphone-equipped system for your home, make sure that it's fully compatible with your smartphone. If you have an Android or iPhone, you can check your settings menu to figure out your software version number and make sure that any software you buy will work with your device. If a manufacturer offers a dedicated app, download the app and see how it works before you buy any hardware.