Dulce de Leche ("milk jam") is Argentina's traditional desert.
It's made over many hours by caramelizing sugar in milk.
There is nothing than can quite compare to this delicious
caramel spread - and I recommend everyone to try it.

It is now available in the United States at some latino stores
that carry Argentine goods.  In addition, it's available by internet
mail-order from a couple of stores. Try Mercado del Plata,
based in California, for delivery all over the world.

When I can't get the real stuff, what I do is buy
one or more cans of SWEETENED CONDENSED MILK
(the low-fat stuff is OK, however), and boil them in water
for about 2 hours. 

BEWARE THAT THIS MAY BE POTENTIALLY DANGEROUS.
The cans say to not boil them, as they may explode, and thus
I can't advise you to follow this method.  I've done it dozens
of times without problems, but you never know.  In any case
it's very important to make sure the water does not evaporate and
add more water if it does.

Alternatively you can try making dulce de leche yourself, but
once again it's a pain in the butt and I've only done it once and 
then lost the recipe.

Use dulce de leche as a filling for the alfajores de maizena.
It's also a great (and traditionaly Argentinian) filling for 
cakes of all types.  Dulce de leche is also commonly spread
on bread or toast (as any jam); toast with butter and dulce de leche
is positively divine.  For an American twist, try it on
bagels or English muffins, or as a topping for ice cream.

You cannot substitute dulce de leche with anything, so don't
even try.

BTW, if you haven't tried Haagendaz new Dulce de Leche ice-cream
flavor, you are in for a treat