The World Bank provides time series data in the World Development Indicators (WDI) data base.
The Center for International Comparisons at the University of Pennsylvania publishes the Penn World Table online. This table provides purchasing power parity and national income accounts converted to international prices for 168 countries. The time series is 1950 to 2000. Access to information on the table and the table download is at http://pwt.econ.upenn.edu/aboutpwt.html . The data is available in csv format.
International Monetary Fund (IMF) -- On its World Economic Outlook web pages at http://www.imf.org/external/ns/cs.aspx?id=29 the IMF provides a series of downloadable reports on the world econmic outlook. A series of world economic outlook databses going back to 1999 is available at http://www.imf.org/external/ns/cs.aspx?id=28 .
The Conference Board provides international time series data at http://www.conference-board.org/economics/database.cfm which includes access to a downloadable flat file of country data going back to `1950.
United Nations -- Data on the national accounts indicators related to the Millennium Development Goals is available at http://unstats.un.org/unsd/mi/mi_goals.asp .