Angus Maddison was an economics professor whose various books - Monitoring the World Economy and The World Economy: A Millennial Perspective - are major works in the field of economic history and contain much historical data on GDP, GDP/capita, and population for countries throughout the world. Since Dr. Maddison's death, the Groningen Growth and Development Centre has continued to update his data. His original site contains links both to excerpts from his books and to much of the original data within them.
Professor Katherine Barbieri at the University of South Carolina has compiled a dataset of national and bilateral trade flows covering the years 1870-2009. This dataset (and others) is freely available for download. Data with coverage up through 2014 are also available via the Correlates of War Project.
China Data Online is a collection of data on economics, demographics, and other topics. Data are available for China as a whole and for individual provinces, counties, and cities. Depending on the topic and geographic area, data are available in annual, quarterly, or monthly increments.
The CPDS were produced by scholars at the University of Berne. They are similar to the Database of Political Institutions dataset but focus on OECD countries and post-communist countries and have more emphasis on demographics, electoral results, and ideological compositions of governments. Related datasets are also available for post-communist countries and for countries that are members of the OECD and/or the European Union. Please note the authors' preferred citation format.
The CWSDS is a joint effort by comparative welfare-state scholars such as Evelyne Huber, Charles Ragin, John D. Stephens, and Duane Swank. The data cover 18 OECD countries from 1960 onwards and include many economic, political, and policy variables, including the structure of political institutions and of wage-bargaining institutions.
Eurostat is the statistical office of the European Union and collects an array of statistical information on member states of the EU and other selected countries (e.g. non-EU OECD members, non-EU members in Eastern Europe). Much of their data are accessible on-line here and cover topics such as population, demographics, balances of payments and economics, agriculture, the environment, and other socio-economic indicators, and much data for sub-national regions within EU member states.
The GGDC is run by the University of Groningen in the Netherlands. It offers a variety of downloadable datasets with economic data for both developed and developing countries (although the most detailed data are for developed countries). The data cover topics such as factor productivity, input-output tables, and national accounts/GDP data for very extended periods of time. GGDC formerly hosted the Total Economy Database, which provides data on GDP, employment, labour productivity, and relative importance of different factors of production to economic output.
Indiastat focuses on India specifically and contains data on a very wide range of social, economic, and political topics.
The ILO is the main international labor-rights advocacy group in the world and has brought together a variety of information about labor rights and working conditions internationally. You can access their databases directly at http://www.ilo.org/global/statistics-and-databases/lang--en/index.htm.
The IMF's data library covers topics such as investment flows, monetary aggregates, economic output, commodity prices, exchange rates, public finances, financial institutions and markets, and both public and private debt. The available databases include the Balance of Payments Statistics, the Direction of Trade Statistics, the Government Finance Statistics, and the International Financial Statistics.
The ITA, situated within the Department of Commerce, is charged with promoting access to foreign markets for U.S. exports. The ITA is a good source for national and state-level data on exports and imports of merchandise goods.
The ITC is a federal agency that conducts investigations into "unfair" trading practices (e.g. dumping) in conjunction with the International Trade Administration. The ITC's website includes resources such as Harmonized Tariff Information which provides access to the tariff schedules for the United States, and the Interactive Tariff and Trade Dataweb, which allows users to query ITC databases on tariffs and trade amounts for specific products.
The OECD iLibrary is an online interface that provides full-text access to OECD studies, periodicals, and the OECD.Stat collection of statistical databases. The topical range of the iLibrary is considerable and covers areas such as agricultural policies, environmental indicators, social expenditures, labor markets, national accounts, foreign trade and FDI, and various industry-level data. While its focus is on wealthier developed countries, the iLibrary also includes OECD reports and publications on selected non-member countries such as the BRIC countries of Brazil, Russia, India, and China.
QWIDS is a relatively accessible interface for very detailed data on foreign aid and development assistance that allows you to break down aid flows by combinations of recipient, donor, and purpose of aid.
For more recent decades, you can get both project-level microdata as well as data on aggregated flows of development assistance.
The Penn World Tables provide data on relative levels of income, output, input and productivity that are comparable across countries and time in PPP-adjusted terms.
TRAINS is a bilateral trade database from United Nations Conference on Trade and Development Statistics that provides product-specific data on tariffs and non-tariff measures. TRAINS is also available via the World Bank's World DataBank and via the Global Database on Non-Tariff Measures.
-- UNCTAD maintains a database on FDI statistics, including flows, stocks, and participation in international investment-related agreements. It covers the years 1970 onward and allows users to download data into Microsoft Excel. UNCTAD's World Investment Reports are another useful source for FDI flows and stocks. UNCTAD's FDI data are part of a database on economic data more generally, including data on macroeconomic indicators, trade flows and trade restrictions, remittances, and commodity prices.
The UN's Comtrade database is a good source for commodity-specific bilateral trade data for the years 1962 onward. Users can create their own extracts of data for both value and quantity of trade using different industrial classification codes and download those extracts for later use. Comtrade now also includes data on trade in services.
The WDI provides convenient access to hundreds indicators on over 200 countries and territories. Data are available annually from 1960 forward, with variation across countries and indicators. The database is part of the Bank's Data Catalog of statistical databases and other data collections. There are also multiples tools available for importing WDI data directly into programs such as R and Stata.
WITS is a joint undertaking of UNCTAD, the World Bank, and the World Trade Organization to integrate various databases on trade flows and trade restrictions. The WITS databases provide information on tariff schedules, non-tariff measures, and bilateral trade and tariff levels. Registration is required to use the WITS interface, but that registration is free of cost. WITS data are also accessible via the Bank's Data Catalog.
The World Bank's Trade and International Integration Group holds several databases on trade and production, trade and services, and tariffs that users can download. The Trade and Production Database may be of particular interest due to its detailed industry-level production, trade, and tariff data. Be warned that addresses of World Bank websites can change without warning; archived copies may be available via the Wayback Machine. Many of the Bank's trade-related data collections are also available via the Bank's Data Catalog and via the Bank's Microdata Library.