Exploring a ruin always increases the resource the nation currently has the least of. You can use this knowledge to get a bonus to a resource of your choice. For example, if you want to increase Food in order to quickly advance to the Classical Age, stop your Scout/exploring unit next to a ruin, then select a City and queue up as many Citizens as necessary, so Food becomes the resource you have the least of. Exploring the ruins at that point will grant bonus Food. Then select the City again and cancel the Citizen building queue to get the Food cost back. Any unit can be used for this trick, but it works best with units that need only one resource to train, like Scholars (Wealth) or Fishermen (Timber).
On Sea maps, an amount of Ruins tends to be located in other Continents; landmasses separated by water. The first nation to build a Dock and send a Scout to that area will have the advantage of access to the Ruins therein; something which can give its nation an edge in an early game.
The Spanish start with an extra Scout unit, their Scouts can see farther and are upgraded automatically. They also receive a slightly higher bonus from ruins, starting with a base amount of 30 and adding 26 per Science level researched.
The Iroquois receive a free Scout whenever completing a barracks, their Scouts are also upgraded for free and can move through forests.