Here are some simple formulas for combining condiments (along with salt and pepper) to create common dressings, which you can amp up with herbs, spices, and other flavorings of your choice to taste.
Ketchup + mayonnaise = Russian dressing
Ketchup + mayonnaise + relish (+ capers) = Thousand Island dressing
Mayonnaise + lemon juice + relish (+ capers) = tartar sauce
Board Links: Salad Dressings