The pamphlet that urged the colonists to declare independence?

There were a number of pamphlets and flyers arguing the political issues in the pre-revolutionary American colonies, but the one that far and away captured the attention of the most people was "Common Sense", published anonymously but actually written by Thomas Paine. He began writing it in 1775, and its 1776 publication was perfectly timed to give it a key role in bringing thousands of people over to the revolutionary movement at a crucial moment.

Once the Revolutionary War had begun, Paine continued writing pamphlets ("The American Crisis" series) to support the cause, inspire the soldiers and encourage the politicians and leaders who guided the difficult struggle.

Incidentally, a significant percentage of the population couldn't read, so the impact of the pamphlets were more than a little dependent upon the oratorical skills of those who read Paine's Words aloud. Even today Paine's writing rings more dramatically when spoken aloud (like Shakespeare's) rather than when trapped flat on the page.