Printables.com (formerly PrusaPrinters.org) – the new object repository database for users of all 3-D printers. [.stl, .3mf, .gcode]
STLFinder – Indexes .stl files from a variety of sources on the internet: a 3-D object search engine.
Cults3D – free/pay object repository.
Thangs.com – Free 3-D model community with geometric searching.
Thingiverse.com – formerly the leader, still a large collection, but site maintenance has gradually declined, owners have added advertisements to site.