What are the most useful sources of economics data? When doing research in Economy, one frequently needs to verify theoretical conclusions on real data. What are reliable data sources to use and cite? I am mainly interested in sources that provide various statistical data such as GDP, population, CPI, PPI etc.
EDIT:
Here's an aggregation of the links appearing in this thread + a few more I remembered.
Generic:
 - Thomson Reuters Datastream (not free, very comprehensive)
 - World Bank Data
 - United Nations Data
 - IMF Data
 - ADB Data
 - WTO Stats
 - Infochimps - massive resource of a wide variety of public and private (commercial) datasources - plus their API
 - Freebase (now owned by Google) - open data resource
 - DBpedia - an approach to using the Wikipedia API
 - Wikipedia API - or go direct and access Wikipedia direct
 - CIA World Factbook
 - OECD Statistics
 - Wolfram Alpha - a knowledge search engine
 - Zanran - a numerical & statistics search engine
 - Inter-university Consortium for Political and Social Research
National:
 - UK government data project
 - US government data project
 - US FRED: Federal Reserve Economic Data
 - US Bureau of Labor Statistics 
 - US Bureau of Economic Analysis 
 - U.S. Census
 - US Health & Retirement Study
 - U.S. Centers for Disease Control
 - U.S. Bureau of Transportation Statistics
 - Eurostat's Databases
Other:
 - OpenData Initiative at Wikipedia
 A: The World Bank data API is particularly good and I wish that more global and state-level organisations would release this much.  Here are a few more to complement @check123:


*

*UK government data project;

*US government data project;

*Infochimps - massive resource of a wide variety of public and private (commercial) datasources - plus their API;

*Freebase (now owned by Google) - open data resource;

*DBpedia - an approach to using the Wikipedia API;

*Wikipedia API - or go direct and access Wikipedia direct;


And the lazy person's choice, there is the CIA World Factbook.  I find that the data is sometimes a bit wrong, but it is a useful place to get a rather plentiful overview.
This is an exciting area of development so expect many more data resources to come.  Follow the Open Data page at Wikipedia for regular updates.
A: In addition to what you've got already, there's http://www.zanran.com/q/ - a search-engine dedicated to numerical data
A: Local/Foreign governments:


*

*Data from Finance Ministry and its bodies

*Reserve Bank

*Official publication of annual accounts of the country


Academic Sources:


*

*Research papers and journals

*Internal archives of universities and institutions

*Dedicated policy and welfare research centers

*Theory/Text books often have further reference


International Aggregates:


*

*World Bank Data

*United Nations Data

*IMF Data

*ADB Data

*WTO Stats

*International NGO(s)

*Print publications from multilateral institutions (like above)


Private Sources:


*

*Research and surveys by local/national and international NGO(s)

*Publications and surveys from mass-media (newspapers, news channels, magazines etc)

*Research and surveys from private organizations (ex - AC Nielsen)

*Publications and reports from financial organizations like Banks, Credit Ratings etc.

A: The U.S. Census Bureau was one of the first government agencies to put data on the web. I still remember the elation I felt back in 1995 when I found out I could get up to date CPS reports and data online instead of having to go through library shelves. They provide both summary tables and public use microdata.
Similarly, U.S. Bureau of Labor Statistics and [U.S. Bureau of Economic Analysis) provide easy online access to both summary and detailed series. BLS's National Longitudinal Surves is used in a lot of empirical micro research.
U.S. Bureau of Transportation Statistics has a lot of tables, but some of them are in quite inconvenient formats. E.g., statistics on boating accidents by the U.S. Coast Guard came in PDF files the last time I checked.
U.S. Centers for Disease Control have an incredible wealth of data on both diseases and behavioral information. Among them is Behavioral Risk Factor Surveillance System which features prominently in health related research these days.
Health & Retirement Study "surveys a representative sample of more than 26,000 Americans over the age of 50 every two years."
A: Rescued from a deleted answer:
If you are interested in the European Union or in some of its member states, you can have a look at Eurostat's databases.
A: For the US:
FRED: Federal Reserve Economic Data (the best) 
Bureau of Labor Statistics 
Bureau of Economic Analysis 
U.S. Census
A: Don't forget http://www.icpsr.umich.edu/
A: For macroeconomic and financial data, Quandl is a great resource, because it effectively acts as a wrapper around many of the excellent sources mentioned here, and many others.
What is more library(Quandl) makes accessing the data in R gratifyingly simple.
A: If you're looking for free monthly global economic indicators to download, have a look at the database on the blog www.morethanbrics.com/blog. They publish a monthly database for up to 169 countries since 1995. I like it because you can download the whole excel file for free and it's updated on a monthly basis. It's based on Worldbank data and includes, among others, the following:


*

*Real GDP Growth

*CPI

*Core CPI

*Industrial Production 

*Retail Sales

*Imports

*Exports

*Foreign Exchange Reserves

*Terms of Trade

*M2 Multiplier

A: There are very related questions in the Economics and quant stacks:
https://economics.stackexchange.com/questions/4679/what-are-some-good-repositories-for-economic-data
https://quant.stackexchange.com/questions/141/what-data-sources-are-available-online
Answers from there: The American Economic Association has a list of resources for Economists, including a page for data, there you find links to many institutions that offer all kinds of data, as well as further journals with data archives for the studies they publish.
In the ReplicationWiki (that I work on) we have information on more than 2000 empirical studies and you can search for which one what kind of data, software, and methods were used, if the material is available, and if replications are known. Many studies can be browsed by JEL codes or keywords. The categorization of data sources and geographical origin of data remains very incomplete but it is a wiki, so everyone can contribute and make suggestions.
