On the eastern side of the Plaza de Mayo stands the Casa Rosada (The Pink House), the presidential house of Argentina (although the President does't live here, and rarely visits).
The Casa Rosada has an interesting history: it started as a fort in 1536 before the city was founded in 1580. The fort was altered and replaced throughout the years until it was replaced with a New Customs House in 1855. In 1873 a Post Office was built nearby, which looked much more impressive and significant compared the older Customs House. Eventually, the two buildings were combined into the building that you see today with a large archway in the middle.
During Domingo Faustino Sarmiento's administration (1868-1874) it was painted pink, and has remained so ever since.
Perhaps the most famous part of the entire building is the balcony, for its role in the history of Buenos Aires. Contrary to many stories, Eva Peron (Evita) never said, "Don't Cry for me Argentina", but she did stand by her husband Juan Peron during his speeches which were given from this balcony over crowds stretching far down the Avenida de Mayo. It was also from this balcony that she told her country she would not run for office, even though they wanted her to, and reveled the unfortunate news of a fatal tumor that claimed her young life and left the city mourning.