USGS Earth Explorer and 3DEP - Satellite and aerial imagery and Digital Elevation Maps at 1, 1/3, and 1/9 arc-second resolution User's Guide
Copernicus - Raster data for Europe, including DEMs, land use, and satellite imagery
The David Rumsey Historical Map Collection - Not all maps are georeferenced, but you can do it yourself with their Georeferencer app!
The Harvard Map Collection - Over 2700 scanned historical maps.
OpenTopography - Direct access to USGS 3D Elevation Program and NOAA datasets as well as derived data products and visualizations (DEMs, slope, hillshade, contours, etc.). Very user friendly.
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