Usually cheese, ketchup, mayo, mustard, and pickles.
Last week in DW, I put the hot melted cheese on my burger at the Pinocchio Village Haus and Pecos Bill's for the first time ever. It was awesome! Now I am hooked. I have been trying to figure out how to replicate that at home. DH says Cheez Whiz, I think that's too strong tasting. Maybe melted Velveeta?