In response to a couple of recent queries, we have significantly improved the answer for:
(The Amercian spelling capitalization is used by the World Bank, one of most respected sources of this information.)
There are a range of sources for this information. Annual summary information is one of the World Development Indicators available directly from the World Bank ( http://data.worldbank.org/ ) or several databases (e.g. ESDS, GMID). More detailed time-series data is available from Datastream.
New question about access to GDP statistics.
This doesn’t cover all the possible sources, so let us know if there are other important ones.