Meshmixer
Free 3D mesh editing for practical model repair
Meshmixer is a free Windows 3D modeling utility from Autodesk built for users who need to modify, prepare, or repair existing 3D models. It is not best viewed as a full blank-canvas design suite, but as a practical mesh editing tool for cleaning up polygon-based objects and making them easier to use in another workflow.
The software supports common 3D formats, including STL, OBJ, PLY, and AMF, which makes it useful for projects that move between different 3D applications.