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"Blender is the free and open source 3D creation suite. It supports the entirety of the 3D pipeline—modeling, rigging, animation, simulation, rendering, compositing and motion tracking, even video editing and game creation." -- website
"MeshLab the open source system for processing and editing 3D triangular meshes. It provides a set of tools for editing, cleaning, healing, inspecting, rendering, texturing and converting meshes. It offers features for processing raw data produced by 3D digitization tools/devices and for preparing models for 3D printing."
"Use the Microsoft 3D Tools powered by Windows 10 APIs to fix 3D files automatically. Like a spellchecker, these tools will save you time by taking care of common errors in 3D files that otherwise would require manual fixing by a dedicated designer." -- website