MeshLab
www.heritagedoc.pt/doc/Meshlab_Tutorial_iitd.pdf
Instant Meshes
www.blendernation.com/2015/11/16/instant-meshes-a-free-qaud-based-autoretopology-program/
github.com/wjakob/instant-meshes
igl.ethz.ch/projects/instant-meshes/
Open3D
www.open3d.org/docs/release/index.html
www.open3d.org/docs/release/tutorial/geometry/pointcloud.html
Point Cloud Utils
awesomeopensource.com/project/fwilliams/point-cloud-utils
MathWorks pcdownsample
www.mathworks.com/help/vision/ref/pcdownsample.html
Not Open Source
Various software
cmacvfx.com/how-to-decimate-lidar-or-photogrammetry/