Population Clock is a nice reference app to show population of the world. The main screen shows it ticking away, whilst also indicating how long until the next major milestone.
You can narrow this down to specific continents/countries, and can present it as a graph over time.
The source of this data is United Nations.
#286) Population Clock
How many people on the Earth now?
Population Clock tell you the real-time population of the Earth.
Concern the Earth, cherish the time, save the resource, and happy life every day!
Including more than 200 countries or regional data. The data source from the United Nations