A Geographic Information System (GIS) "integrates hardware, software, and data for capturing, managing, analyzing, and displaying all forms of geographically referenced information. What is GIS used for? There are many applications, spanning a wide variety of disciplines. Some examples include:
Emergency Services: wildfire spread, missing person location, least-cost paths to accident sites
Social Sciences: crime rates, food deserts
Open Data CensusHere you will find a graphical representation of 70 countries' scores on data accessibility.
The Mid-Atlantic Ocean Data Portal is an online toolkit and resource center that includes a map interface Marine Planner and access to dozens of datasets organized into the following categories; Administrative, Fishing, Marine Life, Maritime Industries, Recreation, Renewable Energy and Security.
Natural Earth is a public domain map dataset available at 1:10m, 1:50m, and 1:110 million scales. Featuring tightly integrated vector and raster data, with Natural Earth you can make a variety of visually pleasing, well-crafted maps with cartography or GIS software. Supported by the North American Cartographic Information Society (NACIS), and free for use in any type of project.
The National Historical Geographic Information System (NHGIS) provides aggregate census data and GIS-compatible boundary files for the United States between 1790 and 2012.
National States Geographic Information Council NSGIC, promotes statewide geospatial coordination activities and advocates for states in national geospatial policy and initiatives, thereby enabling the National Spatial Data Infrastructure (NSDI).
The quality and content of GIS data can vary greatly, as well as the fees and ease of access. Many datasets can be obtained for free. Listed below are links to sources for data types ranging from global to city level.