The International Monetary Fund (IMF) and the World Bank are two of the main sources of economic data at the country level.
The International Monetary Fund (IMF) provides access to numerous reports as .pdf downloads and tables as .csv or Excel downloads at http://www.imf.org/external/data.htm . This page includes links to downloadable data from the IMF's World Economic Outlook. It is necessary to register (no cost) to access some of the data.
The World Bank provides time series data in the World Development Indicators (WDI) data base.The World Bank definition classification of countries by income level (low, lower middle, upper middle and high) is updated annually--the historic record of the levels is at -https://data.worldbank.org/products/wdi-maps.
United Nations -- Data on the financial indicators related to the Millennium Development Goals is available at http://unstats.un.org/unsd/mi/mi_goals.asp .
The University of Groningen publishes the Penn World Table online. This table provides purchasing power parity and national income accounts converted to international prices for 167 countries. The time series is 1950 to 2011. Access to information on the table and the table download is at http://www.rug.nl/research/ggdc/data/pwt/ . The data is available in CSV format.
The Conference Board provides international time series data at http://www.conference-board.org/data/ which includes access to a downloadable flat file of country data going back to `1950.
The International Budget Partnership-- available at (https://www.internationalbudget.org/)has developed an index of budget openness and presents an annual report. The data can be downloaded at https://www.internationalbudget.org/open-budget-survey/downloads/?utm_source=publications#datasets.
The World Input-Output Database provides input output tables for selected countries at http://www.wiod.org/database/wiots16
Aid Transparency Initiative (IATI) IATI is a relatively new project which is developing an online database of international aid flows. The site is available at https://iatistandard.org/en