Free and paid STL files libraries

Thingiverse - http://www.thingiverse.com
Shapeways - https://www.shapeways.com/
Cookie Caster - http://www.cookiecaster.com/
Lithophane - http://3dp.rocks/lithophane/
YouMagine - https://www.youmagine.com/
STLFinder - https://www.stlfinder.com/
My Mini Factory - https://www.myminifactory.com/
Yeggi - http://www.yeggi.com/
GrabCad - https://grabcad.com/
Turboaquid - https://www.turbosquid.com/
Instructables - http://instructables.com/
NASA 3D Resources - https://nasa3d.arc.nasa.gov/
3D Warehouse - https://3dwarehouse.sketchup.com/
Smithsonian - https://3d.si.edu/
ShareCG - http://www.sharecg.com/
All3DP - https://www.all3dp.com/
3DExport - https://de.3dexport.com/
3DContentCentral - https://www.3dcontentcentral.com/
3DShook - http://www.3dshook.com/
3DExport - https://de.3dexport.com/
3DContentCentral - https://www.3dcontentcentral.com/
SketchFab - https://sketchfab.com/
Tinkercad - https://www.tinkercad.com/
Evermotion - https://evermotion.org/downloads/
Blendswap - https://www.blendswap.com/
3DModelfree - http://www.3dmodelfree.com/
Yobi3D - https://www.yobi3d.com/
ThreeDscans - http://threedscans.com/
BlankRepository - https://www.blankrepository.com/
3ddigitaldoubles - http://3ddigitaldoubles.com/
Pixellabs - https://www.thepixellab.net/freebies/
CGTrader - https://www.cgtrader.com/
Pinshape - https://pinshape.com/
XYZprinting 3D Gallery - https://3d-gallery.xyzprinting.com/en-US/gallery/
NIH 3D Print Exchange - https://3dprint.nih.gov/
Redpah - https://www.redpah.com/
Threeding - https://www.threeding.com
Free3D - https://free3d.com
Zortrax Library - http://library.zortrax.com/
Libre3D - http://libre3d.com/
STL Finder - https://www.stlfinder.com/