Sometimes when you're expanding, your cultural borders will cover a lot of real estate that isn't really being used by any city. Is there any mod that will let you see the grids that aren't being used by your cities? I know you can see it when you build a settler, but ideally I'd like to be able to see it BEFORE I decide to build a settler. I used to like the way Civ II would show you the outline of the city, so something like that would be good.

Not really, but pressing [Alt+F] + [Alt+I] + [Ctrl+T] may help you a little to figure out which is and which isn't being used. (Press those again to go back.)

Unless I'm going crazy, I think I saw that option in BtS' options screen. I could quite easily be going crazy though :P

That lets you see it when a Settler is selected.