Digimap https://digimap.edina.ac.uk/ is a map archive including Ordnance Survey, Geological, Marine and Environmental maps, as well as Lidar and Aerial surveys, and historical maps.
Ordnance Survey Open Data: https://www.ordnancesurvey.co.uk/opendatadownload/products.html a resource of different specialised maps, including rivers, terrain, based on OS data.
Jisc Geospatial data https://www.jisc.ac.uk/geospatial-data is a tool that allows for the addition of information and layers to maps.
National Library of Scotland: https://www.nls.uk/digital-resources/data-foundry Digitised historical maps of Scotland.
Google Earth Engine: https://developers.google.com/earth-engine/datasets mapping and imagery from an array of different satellite sensor systems for atmospheric, climate, weather, land use, topography, and other geophysical data.
Esri/ArcGIS Living Atlast of the World: https://livingatlas.arcgis.com/en/browse/#d=2 Imagery, base maps, and maps for population, infrastructure, and environmental information.
USGS Earth Explorer: https://earthexplorer.usgs.gov/ Satellite and aerial imagery, vegetation monitoring, digital topography, and much more.
ESA Copernicus Open Access hub: https://scihub.copernicus.eu/ access to data from the Copernicus satellite program, including land use, water quality, land cover classification.
OpenTopography: https://opentopography.org/ high resolution topography data and tools.