Microsoft 3D Model Repair (netfabb)
"Use the Microsoft 3D Model Repair service powered by netfabb to repair STL, OBJ, or 3MF files automatically. Like a spellchecker, this service will save you time by taking care of the many common errors in 3D models that otherwise would require manual repair by a dedicated designer. The service closes holes in the model, fixes face normals, removes self-intersections, and more." 100 MB limit on file size.