GADM -- worldwide administrative boundaries (more detailed than Natural Earth, larger files)
Openstreetmap - online map, crowed-sourced... exported data can be used in QGIS, but not ArcGIS
Bureau of Land Management -- public lands data
California State Geoportal - maps, apps, and data from over 30 state departments
United Nations Environmental Programme -- spatial and non-spatial data on a variety of themes
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