When I first got into this i scoured google earth for the signs. I sent them what I believe to be the ruins of probably the worlds largest fort by like a lot near Pompeii but I don't think that's quite up their alley. I'll see if I can find my screen shots see what you guys think.Thanks for the new links.
Starforts.com is an OK source for beginning. It is NOT exhaustive. It is missing quite a few even in New York City