If we're talking real, live BBQ--meat cooked low and slow on a smoker--I love cole slaw, especially with pulled pork. I grew up eating pulled pork sandwiches with the cole slaw on the sandwich. Great stuff.
If by BBQ you mean grilling, then I really like grilling fresh corn on the cob, and also like doing a steamed veggie packet on the grill, too. Lay out some foil and put on it some sliced zucchini, onions, peppers, mushrooms and add a little minced garlic. Salt/pepper, then add a small dab of butter and a little white wine as a steaming agent. Fold up the foil into a packet, add a couple of small holes for steam to vent, then put on the grill for 10-15 minutes.
And fruit is really good grilled, too. Peaches, nectarines, apples, oranges, pineappple, etc. get nice and caramelized, especially if you add some butter (can you tell I like butter?

) and a little brown sugar. The slightly smokey flavor and caramelization go really well together.
Whether you're smoking or grilling, baked beans go great with both and a must-have for me.