The East German “Peaceful Revolution” of 1989: Non-violent, weekly demonstrations led to the fall of the Berlin Wall and resulted in the unification of Germany.
The March on Washington of 1963: 250, 000 people poured onto the streets to demand civil and political equality for African Americans- a pivotal moment in the American people’s struggle to end overt racism and inequality.
Tiananmen Square, 1989: About one million Chinese people who peacefully occupied Tiananmen Square for nine weeks were suddenly met with army tanks, with fatal results. Change has come at snail’s pace in China, but should these brave protestors have returned home and continued to live under Chinese dictatorship and not fought for freedom?
Revolution against the Revolution, June, 2009: The people of Iran demonstrated in massive numbers after the allegedly fixed elections of that same month. They were met with curfews, bullets, arrests, beatings and a couple of executions. The people did not get what they wanted then, but they are back on the streets now, in Jan-February of 2011.
The Storming of the Bastille in 1789: So this is not the most peaceful form of mass or popular resistance (afterall, the guillotine is famous from this portion of history). However, the relatively small demonstration did mark the beginning of the French Revolution, the beginning of the end of French monarchy, and the eventual democratization of France.
For a very detailed list of protests and similar events that changed the course of history, see Google’s outline from 1500 to 2011: