Models
BlenderKit add-on (for 2.8, commercial but with some free stuff)
Chocofur Wide variety of high quality blend files made for Cycles and Blender; freebies at this location
Noob to Pro (list of resources)
Sketchfab Downloadables Large collection; large variety; various licenses; blend files are far and few in between but formats that Blender can't import are rare too
TurboSquid Registration required; large collection of free stuff; blend files rare
3D Model HavenPolyHaven Expanding library of high quality models as blend and fbx files (will soon be merged with other Haven sites > https://polyhaven.com/)