1. Car charger
Do you drive? Probably. And it's also fairly safe to assume that you're not particularly happy with your smartphone's battery life, either. Especially if you're on the road and need power for navigation. That's why a car charger is among the most essential accessories to have around. Most of them are dirt cheap, though we do recommend you get your hands on something branded. It also helps if it has an extra USB port -- passengers need power, too!
2. Car dock
So we've already established that you drive, meaning that you also need navigation-at-a-glance fairly often. Can't get that with a smartphone stuck in your pocket, and that's where the handy car docks come in. Many of those are suction-based (they stick on your windshield), and some are specifically built for your exact model, even though there are quite a few universal ones. They're also, once again, a pretty cheap investment in comfort and peace of mind.
Oh, and by the way, a car charger is almost a must if you use navigation -- the GPS and 3G/4G LTE modules alone will drain your battery in a few short hours.
3. Power banks
A power bank, much like a car charger, can be a lifesaver, especially if you're far from your vehicle. Best of all, these occasionally come in shapes so small, that you can easily carry them on your person at all times without much discomfort.
4. Bluetooth headset
So you're back in the car, eh? You probably could have walked there, but we won't judge. In any case, a light-weight, unobtrusive Bluetooth headset can literally be a life-saver. Best of all, a Bluetooth headset comes in handy in more than just car scenarios -- just try picking up an important call whilst holding 5 bags of groceries and you'll see what we mean.
5. An MHL/SlimPort microUSB to HDMI adapters
This one's really cool if you have a big-screened TV that lacks the smarts of your phone. Instead of having to go to the trouble of connecting a full-fledged laptop or computer in order to watch some Netflix, you can get the same result by connecting your smartphone to a TV's full-sized HDMI port through an MHL or SlimPort adapter. These two standards are becoming more widely-supported by the month and allow you to stream content right from your smartphone.
6. Chromecast or Chromecast-like dongle
Cables are a nasty business, and making your smartphone a slave of your TV is not the best decision out there. Enter Chromecast -- a simple USB dongle from Google that connects wirelessly with your phone, allowing you to stream content from it on your plasma. You can pick one of those for $30 nowadays, which makes them all the more awesome.
7. Wireless charger
Building upon the notion that cables just aren't sexy anymore, the industry gave us wireless chargers. Simply pop your smartphone on top of one of those, and watch as your battery charge shoots up. Sure, not all smartphones have support for wireless charging, but the list is growing, and some of them make for some seriously cool desk docks. Two birds with one stone, poof!
8. Extra battery
Wireless chargers are cool, but they still need a traditional power source. And power banks? They're not that small.
But if your smartphone's battery is removable and you're constantly power-starved, then picking an extra battery for your model may be a good investment. A Samsung Galaxy S4 replacement battery can cost as little as $10, for example, and it's small and slim enough to fit in even the tightest of pockets. Just make sure you pick an original one, we've had enough reporting on low-quality units catching on fire.
9. OTG, or USB On-The-Go
Even today, most smartphones come with just a few gigs of available space. That's clearly insufficient space for your library of 10,000 songs and 1,000 movies. Really spacious microSD cards are still rare and expensive, and while they're arguably a much more useful accessory, they sadly aren't supported by every phone model out there. That's where USB OTG (On The Go) cables come in play, and while support for the standard still isn't ubiquitous, it is growing. USB OTG is a simple, pocketable microUSB to a full-sized female USB cable, and it allows you to attach peripherals such as keyboards, mice, and so on. Going back to storage, you can pick a 128GB flash drive for less than $30 nowadays, and that's plenty of room for your music and movies.
This last item on our list is a bonus, as it's not a category, but rather a very specific and unique accessory. Pressy received massive backing on Kickstarter, and it basically acts as an extra hardware button to your smartphone. You simply plug it in your (Android) phone's 3.5mm audio jack, and download the Pressy app available from the Google Play Store. You can then use the app to customize the behavior of Pressy -- it can turn into a shutter key, a flashlight shortcut, record a phone call, and tons more. Pressy is currently available for pre-order at $27, and the team has promised a March release.