I put salt, pepper, vinegar (malt vinegar is best) and mustard on my fries. I'm the only one in the house who eats them that way, unless my eldest brother is visiting, so my fries rarely get stolen.
I'll also been known to eat fries with thousand island dressing as a dipping sauce. Ketchup is OK, but not my fave. In college, we used to put chili, cheese, and mustard on our fries. That was back when I could do that and still fit into my pants the next day!