Links to potentially useful data and spatial information, most of it freely available. Some data may need registration. This list doesn't include data available via Digimap or similar EDINA services.
Extremely useful open source mapping of the whole world at general scales. Useful for whole-world or country-wide background mapping.
If you're not sure how to get started with this data try the "Quick start kit" which includes styled data in an ArcMap mxd document and a QGIS project. Download the approximately 160 MB zip file from the Natural Earth downloads page.
The map equivalent of Wikipedia. Can be extremely useful, but be aware of possible inaccuracies! The data can be edited by anyone - it's the equivalent of Wikipedia for maps. Some areas are very sparsely covered. It tends to depend on whether an OSM enthusiast lives in or has visited an area. Of course, you could always edit it for yourself. The big advantage is that you can use the mapping under an Open Data Commons Open Database licence as long as you credit "© OpenStreetMap contributors".
Vector data for limited areas can be downloaded from the OSM site and imported into various GIS software including Arc. To download shapefiles for entire countries or regions try Geofabrik. Once you have the data you will need to style it in some way. This is probably most useful if you just want, for example, the rivers for an area for which you can't find any other data.
Administrative boundaries for all countries of the World.
Long list of free datasets. Plenty of world-wide thematic data, such as population or climate. Also has section on country-specific data. Worth applying the warning at the top of the page to all GIS data!
A searchable and browsable list of a wide range of datasets. Some are background layers, but there are also data layers, such as earthquakes.
Directly viewable in ArcGIS Online from links on the pages.
To find out how to use living atlas layers in QGIS, view the tutorial by Geography Realm.
In ArcMap you can use layers from ArcGIS online as backgrounds for your maps. Do check these print, though, they don't always!
In QGIS there are some plugins which provide access to web mapping, such as OpenStreetMap. Try QuickMapServices, or the OpenLayers plugin.
These are places where you can find similar data to Digimap for the UK, but open source versions. Some of the same data is available, but not usually the more detailed. You may need to register to download data. Read the licences/terms of use to see how you can use this data.
The less detailed maps are available, e.g. 1:250 000 rasters, but also Vector Map District in both vector and raster format. Contours and DTM are available as OS Terrain 50.
A wide variety of data, though downloadable geology for GIS is only available at 1:625 000.
Viewer for maps and sections published between 1832 and 2014. Also links to their other data viewers.
Lidar data is available separately for England, Wales and Scotland. See the links in the Height and satellite data section.
The data from the Environmental section of Digimap. You can download 1 km raster data from this URL. Anything else needs to be licenced. There are a lot of other data sets available from these pages too.
Available layers include ASTER Global DEM v2 and SRTM data from NASA (useful as DEM for larger areas), also Landsat data. You'll need to register but there is a lot of useful data available. Once you've registered, it is also possible to download SRTM data using the SRTM Download plugin for QGIS (Link goes to a tutorial).
Lidar data for England and Wales is now available in Digimap, but it can also be obtained directly via the links on the page above.
Provides access to remote sensed data provided by the Scottish Public Sector. At the moment this basically means Lidar height data. Coverage for Scotland is very limited.
Country-level geological mapping. Not complete but increasing in coverage. Mainly available through a web viewer, but also possible to find WMS urls which can be used in GIS. Follow the link to OneGeology Portal
The Geological Survey of Japan maintain a comprehensive list of Geosciences organisations which provides a very useful starting point for finding geological information on-line.
Viewer for maps and sections published between 1832 and 2014. Also links to their other data viewers.
Geologic mapping for the United States. Most is freely available to download. Includes scans and data for GIS.
Various datasets to download. 1:250 000 geological maps are available as non-georeferenced scans.
Free registration required, but then a large number of downloadable datasets including height data, scanned and georeferenced maps, and imagery.
Search and browse for vast amounts of Canadian data. This is a central repository, but it is also worth searching for data from individual states, e.g. Yukon has GIS-ready data available to download.
Access to a lot of maps and other data sources. A lot of the USGS publications can be downloaded for free.
Free registration required.
A lot of data is freely available. Follow the download geodata link to see the full range. If you're working in QGIS, you can use the Swissgeodownloader plugin to download Swiss data.
A lot of data is openly available. Full list in the Map Catalogue.
Includes population statistics, but also related statistics such as employment etc.
Wide range of datasets available to download. Mainly datasets which have been requested, but some derived from the census.
Population data covering Asia, South America and Africa.
Find a coordinate system and get positions on a map. Allows searching by country, coordinate system or epsg code. Also allows you get coordinates for a point on a map in any coordinate system, or to convert coordinates for a point between one system and another, e.g. WGS84 to British National Grid or vice versa, and also to change degrees, minutes, seconds to decimal degrees.
Extensive list of coordinate systems which can be useful to find or confirm when projecting in GIS.
UN Data Commons provides data and maps for Sustainable Development Goals. Data can be browsed on the web, but much of it is also available to download.
Free and open access to biodiversity data. Contains point data for an amazing number and diversity of systems. Free registration required.
Downloadable data for more than 88,500 endangered species.
A free, online database of animal tracking data hosted by the Max Planck Institute of Animal Behavior
Be aware that often, when someone talks about land use data, what is actually available is land cover data.
Land cover data for Europe from Copernicus. Data is available back to 1990.
Worldwide land cover mapping based on Sentinel satellite data.
A blog which covers a wide range of open source data.
A very wide-ranging list of links to GIS data. Note the warning at the top of the page!