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.
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.
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 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.
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.