Our top three would be:
Thingiverse – http://www.thingiverse.com/
PrusaPrinters – https://www.prusaprinters.org/
My Mini Factory – https://www.myminifactory.com/
Other sites
123D App – www.123dapp.com/gallery/content/all
3D Cab Browser – www.3dcadbrowser.com
3D Warehouse – https://3dwarehouse.sketchup.com/
3DContentCentral – https://www.3dcontentcentral.com/
3ddigitaldoubles – http://3ddigitaldoubles.com/
3DExport – https://3dexport.com/
3DExport – https://de.3dexport.com/
3DModelfree – http://www.3dmodelfree.com/
3DShook – http://www.3dshook.com/
All3DP – https://www.all3dp.com/
Archive 3D – www.archive3d.net
BlankRepository – https://www.blankrepository.com/
Blendswap – https://www.blendswap.com/
CG Trader – https://www.cgtrader.com/
Cookie Caster – http://www.cookiecaster.com/
Cults3D – https://Cults3d.com
Digible – www.digible.net
Evermotion – https://evermotion.org/downloads/
FAB365 – https://fab365.net/
Free 3D – https://free3d.com
GrabCad – https://grabcad.com/
Gumroad – Gumroad.com
Instructables – http://instructables.com/
Libre3D – www.libre3d.com
Lithophane – http://3dp.rocks/lithophane/
NASA 3D Resources – https://nasa3d.arc.nasa.gov/
Open 3D Model – www.open3dmodel.com
Pinshape – https://pinshape.com/
PinShape – www.pinshape.com
Pixellabs – https://www.thepixellab.net/freebies/
Print a Brick (Legos) – https://printabrick.org
Remix3D – https://www.remix3d.com/
Repables – https://repables.com/
Shapetizer – www.shapetizer.com
Shapeways – https://www.shapeways.com/
ShareCG – http://www.sharecg.com/
SketchFab – https://sketchfab.com/
Smithsonian – https://3d.si.edu/
STLFinder – https://www.stlfinder.com/
ThreeDscans – http://threedscans.com/
Tinkercad – https://www.tinkercad.com/
TreatStock – www.treatstock.com/3d-printable-models
Trinpy – www.trinpy.com
Turboaquid – https://www.turbosquid.com/
Yeggi – http://www.yeggi.com/
Yobi3D – https://www.yobi3d.com/
YouMagine – https://www.youmagine.com/