General data sources

Our World in Data: open-access data source for a wide range of indicators
World Bank Open Data: open-access data source related to development
OECD data: open-access data source primarily on advanced economies
Eurostat: European Union data
Gapminder: open-access data source related to development; all World Development Indicators also available here

Field-specific data sources

Global Biodiversity Information Facility (gbif): citizen science based open-access database containing information on species occurrences all across the world
EDI Data Portal: collection of environmental data and metadata derived from publicly funded research
Ecological Data Wiki: Wikipedia of ecology data
International Institute for Sustainable Development- Experimental Lakes Area (IISD-ELA): biological, environmental and meteorological data on freshwater lakes and watersheds
European Soil Data Centre (ESDAC): biological, physical, and chemical information on the European soils
DRYAD: open-access research data
eBird: data collection on bird distribution, abundance, habitat use, and trends
waterinfo: physical and chemical data on water in the Netherlands
British Antarctic Survey: database on agriculture, atmosphere, biosphere, climate indicators, cryosphere, human dimension, hydrosphere, land surface, oceans, paleoclimate, solid earth and sun-earth interactions in the antarctic region
WorldClim: database of high resolution global weather and climate data including future predictions (spatial data)
Land-Use Harmonization: database of high resolution land use information (spatial data)

Total Economy Database: data on GDP and labour statistics for 131 countries (available after creating a free account)
FRED (US Federal Reserve): mostly US economic data
Maddison Historical Statistics: GDP per capita and population data for 169 countries between 1000–2018
DBnomics: collection of publicly available economics data