The Plaza de Mayo in the very center of the city, has a rich historical background, starting with it's namesake event, the begining of the May 25, 1810 revolution against Spain that eventually led to independence. It is flanked on three sides by important historical buildings. The lovely Cabildo was the seat of government and the voice of the King during colonial times. Now, the Casa Rosada, the Pink House, is the seat of the president and goverment, headed by the once popular Cristina Kirshner. The third side is flanked by the Metropolitan Cathedral, a huge church that looks more like a congress building than church, as it has no indentifying tower, statues, or domes.
In the center is the Obelisk and the hub of political activity in Buenos Aires. Whenever an issue comes up, this is where it is protested or supported. It is famous as the site of the Mothers of the Plaza de Mayo's marches, which began during the military dictatorship in which 30,000 people were murdered and disappeared. They were the most vocal and visible opponents to the regime, and demanded information about their missing children. They still march every thursday at 3:30pm.
You will always find political banners and slogans hung in the plaza, along with vendors who sell flags and Argentina mementos and groups of activists. It's arguably the most famous site in Argentina.