Museums & Cultural Institutions
Smithsonian 3D Digitization - https://3d.si.edu/
Europeana 3D Collections - https://www.europeana.eu/en/galleries/15694-twin-it-a-pan-european-collection-of-heritage-3-d-models
Science, Space & Government
NASA 3D Resources - https://nasa3d.arc.nasa.gov/
NIH 3D Print Exchange - https://3dprint.nih.gov/
USGS 3D Elevation Program - https://www.usgs.gov/3d-elevation-program
MapToModel - https://www.map2model.com/
Academic / Research Repositories
MorphoSource - https://www.morphosource.org/
Digital Morphology - https://digimorph.org
General
MakerBuildit - https://makerbuildit.com/collections/stl-files
MakerWorld - https://makerworld.com/
Thingiverse - https://www.thingiverse.com/