Regionally, it's typically ketchup, mustard and relish, or your combination of choice. Raw red onions are common too.
Me, I typically go ketchup and mustard when just the "staples" are available.
But I love chilli cheese dogs, pepperoni on dogs is fantastic, bacon (REAL bacon, hehehe) is good on everything, sauteed peppers and onions, the hot dog cordon bleu is a delight, and right now I'm on a sauerkraut and mustard kick.