Kindle Fire Factory Cable
I'm a little surprised nobody has mentioned the factory cable in the accessories section. For anybody doing anything more than using the Kindle Fire in the stock configuration, I think the factory cable is a must-have accessory. It makes so many tasks so much simpler to perform. Getting out of bootloops, flashing a ROM for the first time, or unbricking... they all get reduced to at most 2 or 3 steps... all without any need to troubleshoot. If everybody had a factory cable (and knew how to use it) before they started tinkering with their device, the general section wouldn't be packed full of help requests.
For those who are unfamiliar with the factory cable, it's a special USB cable that will put the Kindle Fire directly into fastboot mode when it starts up. This is important because the hardest part of fixing most problems on the Kindle Fire is first getting the device into fastboot mode. Even in the worst bricking incidents... if you have any working bootloader, use the factory cable to get into fastboot, flash a new bootloader/recovery/ROM, reboot and the thing is fixed. The less familiar you are with how all of this stuff works, the more useful the factory cable will be in your toolbox.
You can make your own from a generic USB cable if you have a soldering iron and some time. I personally could have gone the DIY route, but I decided to purchase one from a fellow XDA member. He makes them himself and really takes pride in the quality of his products.
It's saved me once and been a great timesaver in many other instances.