Pokemon GO developer Niantic declared in a press release that the wrist watch-like accessory Pokemon GO Plus will be available "in much of the world" on September 16th. This device will apparently use Bluetooth Smart to connect with players' phones, letting them capture Pokemon and collect items without having to look at their phone.
Niantic went into further details about how Pokemon GO Plus will function, stating:
"As you pass by a PokéStop, Pokémon GO Plus will vibrate and light up and alert you to the location. You’ll even be able to collect new items just by clicking the Pokémon GO Plus. If there’s nearby Pokémon hiding in your area you’ll be alerted with lights and vibrations; catch the Pokémon with a button on Pokémon GO Plus and then continue on your way (later, you can check your Journal to see which Pokémon you’ve just caught)."
While Pokemon GO Plus will be available to most countries on the 16th, several South American countries will be waiting until a later date to receive the device. Niantic has not gone into details about the reasoning behind this delay.