If I remember correctly, Civilization 4 offered some technology that revealed the entire minimap. It didn't break fog of war, but at least it allowed you to see where all the cities were and what the terrain was like.
So far in Civilization 5, I haven't seen any equivalent to that. I just decided to have a go on the largest size map, and exploring has been daunting, to say the least. Is there any technology I can keep in mind that will fill in any gaps that are left from my exploration efforts?