Dust3D - Cross-platform open-source 3D modeling software.
Google Poly - Explore the world of 3D.
Polyscope - C++ viewer and user interface for 3D data like meshes and point clouds.
Open3D - Modern Library for 3D Data Processing.
Spoke - Easily create custom 3D environments.
Buerli - Browser based modeling. Non-manifold, programmatic CAD workflow for the web.
hmm - Heightmap meshing utility. Convert a grayscale heightmap image into a 3D mesh.