Horseradish in mayo or Heinz57 sauce, usually. I like something with a bite to it. A good whiskey sauce is nice too. A1 and Heinz57 only differ in their base. Heinz has apple puree, and A1 uses raisin puree for the sweet component in the sauces, but the other big flavor ingredient in both sauces is anchovies, believe it or not! They're very salty and oily things and rendered down for their flavor in the sauces, used a lot like bacon is in sauces and as a flavoring.
I hate to tell you, but even gourmet cooks utilize ketchup in recipes...it's just a very tomato-y version of barbeque sauce, tomato sauce, sugar, vinegar and spices. Chefs used to make their own and it was so popular it finally got bottled for convenience sake in the 1910 era.
Meatloaf without ketchup is a sorry thing, if you ask me.