Quandl - GDP by country - downloadable in CSV, Excel, JSON or XML
Historical U.S. GDP (yearly data), 1790–present, maintained by Samuel H. Williamson and Lawrence H. Officer, both professors of economics at the University of Illinois at Chicago.
Google – public data: GDP and Personal Income of the U.S. (annual): Nominal Gross Domestic Product
The Maddison Projectසංරක්ෂණය කළ පිටපත 2013-04-29 at the Wayback Machine of the Groningen Growth and Development Centre at the University of Groningen, the Netherlands. This project continues and extends the work of Angus Maddison in collating all the available, credible data estimating GDP for countries around the world. This includes data for some countries for over 2,000 years back to 1 CE and for essentially all countries since 1950.