Ketchup I mainly use on hamburgers. If the fries are good enough they don't need anything but salt, though sometimes I use vinegar too. Mustard is my go-to condiment.
I like buttermilk if its tangy, but some brands have a more yogurty, chalky taste I don't care for. I sometimes crumble cornbread in a bowl and cover it with buttermilk for a light supper.
And Malcolm Gadwell has an interesting article on ketchup.
Mustard now comes in dozens of varieties. Why has ketchup stayed the same?