Nowadays, smartphones come without physical buttons, but if you need to have at least one button with a dedicated function, you might want to check out this device.
If you want to add one extra button to your Android device, you might be interested in Pressy. Pressy connects to your smartphone or tablet using headphone jack, and thanks to the Pressy app, you can assign specific function to it. This can be perfect if you need one button for your camera or if you need a button to bring back quickly the home screen. At the moment, Pressy is a Kickstarter project, but we should see it available in near future.
If you need one extra physical button on your Android device, Pressy might be just what you need. You can support Pressy by donating $17 to its funding, and as for availability, Pressy has reached its funding goal, so we should see it available in near future.