Open source 3D creation. Free to use for any purpose, forever. – Free
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, video editing and 2D animation pipeline.
Microsoft 3D Builder – Free
View, create, and personalize 3D objects using 3D Builder.
Download many kinds of 3D files and edit them using wide range of powerful yet easy-to-use tools. Take a picture with your webcam and make it 3D. Personalize 3D objects by embossing your name on them, or combine models and pieces to make something new. Build from scratch using simple customizable shapes.
Mesh Maker – Modeling & Editing Apps
With your purchase of Mesh Maker you get all fourteen modeling & editing programs in one bundle. They are all designed for creating new meshes and editing meshes within the Unity editor. You can launch these programs one at a time or use them in combination to greatly enhance the power of Unity.