Thingiverse    

https://www.thingiverse.com/

Grabcad         

https://grabcad.com/

Sketchfab       

https://sketchfab.com/

Autodesk 123d           

http://www.123dapp.com/

CGTrader Marketplace   

https://www.cgtrader.com/

My Mini Factory         

https://www.myminifactory.com/

Yeggi  

https://www.yeggi.com/

Pinshape        

https://pinshape.com/

XYZprinting 3D Gallery          

http://us.gallery.xyzprinting.com/

Dremel Idea Builder  

https://3dprinter.dremel.com/3d-printing-models

YouMagine     

https://www.youmagine.com/

Yobi3D

https://www.yobi3d.com/#!/

STL Finder

http://www.stlfinder.com/

3DExport        

https://3dexport.com/

Cults   

https://cults3d.com/en

Zortrax Library

http://library.zortrax.com/

New Matter   

https://store.newmatter.com//#!/designs

Rinkak

https://www.rinkak.com/

3DShook         

http://www.3dshook.com/

Threeding       

https://www.threeding.com/

Repables        

https://www.redpah.com/

Libre3D           

http://libre3d.com/

3Dagogo         

https://www.3dagogo.com/

Shapetizer      

https://www.shapetizer.com/

The Forge       

http://zheng3.com/forge/index.php?id=-4

Redpah           

https://www.redpah.com/

Trinpy 

https://www.trinpy.com/

Polar Cloud     

http://cloud.polar3d.com/

3D Warehouse           

https://3dwarehouse.sketchup.com/index.html

STLHive          

http://www.stlhive.com/

NASA  

https://nasa3d.arc.nasa.gov/models/printable

NIH 3D Print Exchange           

https://3dprint.nih.gov/

Instuctables   

http://www.instructables.com/

TinkerCad

https://www.tinkercad.com/things/