Sandwich bread (1 slice per rarebit)
Cheese (we always use mild cheddar—enough to cover each slice of bread)
The sauce (we always use a combination of soy, tobasco, and taco sauce. I couldn't tell you what my great-grandparents use)
Take whatever ingredients you'd like to add, and mix together to make the sauce. We usually use about 45% soy sauce, 45% taco sauce (whatever we happen to have on hand) and about 10% tobasco sauce (for a bit of a kick). My father loves throwing in minced garlic, as well, and one can add just about anything one likes to the sauce to flavour it as desired.
Spread out bread on a cooking sheet, add cheese, and spread sauce (with a spoon, spatula, butterknife, or similar impliment) over the cheese. One variation that has become popular in my family is to add tomato slices on top of the sauce (before or after cooking).
Cook at about 350 until cheese is melted.
As above, but toast bread before preparing—this helps the bread from getting too soggy. And make sure to use a microwave-safe plate. Microwave on high until cheese is melted. It should only take 20-30 seconds per rarebit (depending on microwave tempuratures, etc.)