In terms of small stuff:
Make or break a merchant by helping or refusing to help him with pirates.
End up helping or hurting the necromancer some time before they even learn of him by supporting different churches with different burial practices.
Save a village from locathah, sahuagin, or the like.
Alter prices of a particular good by an adventure in the source of that good which ends up increasing or decreasing its supply.

Big stuff:
Reveal the area's location to an imperialistic power such as Amn, or a mighty dragon of some sort.
Overthrow the area's government on their own, setting the PCs up as a ruling council.
Convincing different islands to revolt and become independent.
Introducing or eradicating certain forms of worship within the island.