You already put some of them, so I'm going to add:
Gapminder This is a database created in Sweeden by Professor Hans Rolling. Gapminder uses the data of the several sources such as World Bank, United Nations and their subsequent institutions, the International Monetary Fund, World Health Organization and several more. Also, there's a interactive software which is free to download and use it to your investigation.
Quandl You can always upload your own data in R for example, but there's a second choice which offer Quandl, a big and large database containing critical info about stock market, bond price, climate change, health, population wealth and more topics. All you have to do is download the library, available in the website linked before and use it.