I added too much water to my spaghetti sauce and I am hungry ! What's the best/quickest way to thicken it up ?

Pour it through a fine sieve to extract some of the liquid. Over cooking may alter the flavor of some ingredients.

But why were you adding water to your spaghetti sauce in the first place? Certainly non-essential.

Pour watery sauce into a non-reactive oven proof vessel {lasagna pyrex, roasting pan, large skillet} and "bake it," at about 350F - 400F stirring it with a wooden spoon or heat-proof spatula every 10 minutes until it's at desired thickness.

I stir in grated parmesan just before serving as long as it's tomato based and I'm passing the cheese at the table.

If it's a long cooking sauce you could boil it down. Or how about adding some tomato paste or some fresh breadcrumbs?

I would just turn the heat up and cook the sauce until the extra water evaporates! Might take a few minutes longer than adding a thickener, but it would be the best way to avoid changing the flavor of the sauce. :)