This will be a short list of the tools you need to have if you are going to be a true pit master and turn out the best possible BBQ product. Since this site is all about electric smokers I'm going to go a head and assume you already have a smoker. You can check out the reviews on here to find the best model for your cooking needs.
Tongs and Spatula - These are absolutely essential for obvious reasons. You are going to need to pick up, flip over, or move around steaming hot meats and you don't want to use your hands! Make sure you get long handled durable versions, since some larger items you are smoking will be too much for your regular kitchen spatula and tong sets.
Remote thermometer - Preferably one with a meat probe that is wireless, or has a high temp wired connector. You can stick this into a piece of meat and then sit back and wait for it to get up to a set temperature when you know its done. Having a meat thermometer takes a lot of guess work out of cooking and allows you to produce top quality bbq everytime.
Meat injector - If you want that deep down cooked in flavor a meat injector will do it for you every time. Get one with a wide tip so you can inject herbs as well as marinades, and even whole cloves of garlic!
Basting brush - Sometimes called a "sauce mop" you want a brush that can layer on bbq sauce or other marinades as you cook, or right after meat has come off the grill.
Large cutting board - If you cook brisket or ribs you need a large surface to chop up your finished product. I recommend one at least 30" across and made out of plastic or wood. Many have a channel that runs along the outer edge and will prevent dripping onto your counter tops.
Those are my 5 necessary tools for cooking bbq. Of course there are hundreds of variations of each, and many other tools out there, but if you follow this list you will be well on your way to pitmaster status!