Wikipedia
Quandl
Quandl is a search engine for numerical data. The site offers access to several million financial, economic and social datasets. Quandl indexes data from multiple sources allowing users to find and download in various formats. All Quandl's data are accessible via an API. API access is possible through packages for multiple programming languages including R, Python, Matlab and Stata.
An Excel add-in allows access to data, including stock price information.
Quandl's sources include the UN, Worldbank, Eurostat and several hundred more.