British Library Maps - Maps from the British Library that have been digitized and made available. Includes various themes and time periods. You can also explore their Flickr Commons collection, and find (or assist!) georeferenced maps with their Georeferencer.
Esri Living Atlas - A wide variety of openly accessible geospatial data created by the Esri community, including thematic and topographic data, and much more.
Landsat data - Available through Esri's Living Atlas, this platform allows users to select satellite data by year, going back to at 1978. Users can select the extent, and export imagery as a .tif. More information can be found on Esri_landsat's ArcGIS Online page.
Map Search - National Library of Australia. A search tool with more than 13 500 maps.
OpenSeaMap - like OpenStreetMap, OpenSeaMap shows the current waterways of the world, using freely available data to show weather (wave height, temperature, wind, precipitation, NOAA data), sea marks, ports, marine traffic, bathymetric lines, and more.
USGS transportation data - This includes line and point data for each state, divided by transportation type, including airports, railways, comprehensive road systems, and trails.
The National Map (U.S.) - From USGS: "The National Map is a suite of products and services that provide access to base geospatial information to describe the landscape of the United States and its territories. The National Map embodies 11 primary products and services and numerous applications and ancillary services."
Online Guide to U.S. Map Collections - This ArcGIS Online hub includes information on the various map collections in each each state.