Today I discovered geoBoundaries, a CC BY 4.0-licensed database of political administrative boundaries covering the entire world. It is notable for its high level of detail, going from ADM0 (country), ADM1 (states/provinces), ADM2 (counties/departments or municipalities), to ADM3 (municipalities or sub-municipalities) for many countries. My go-to source for world map files is Natural Earth, which is limited to ADM0 and ADM1 but is in the public domain. Natural Earth also includes some physical geography like water and bathymetry, while geoBoundaries is focused solely on political administrative boundaries. Both datasets deal with disputed boundaries, which is an endless source of tension in the Natural Earth GitHub.
An R package for retrieving data from geoBoundaries, geobounds, was released in February. A similar package for Natural Earth, rnaturalearth, has long been maintained by rOpenSci.