well i’ll try, starting with the standalone freeware:
CloudCompare for working with pointclouds, tasks like cleaning, smooting, filtering, registering scans, normal generation, analyzing cloud/cloud or mesh/cloud deviations, mesh generation from point soups etc.
MeshMixer for checking meshes, cleaning dents, filling holes, smoothing, refinement, decimation, segmentation and overal inspection of meshes. Also used to make shells for quick printing and sculpted changes, booleans etc.
The PlugIns (add-ins as you say) mentioned above used in Rhino (not free) and their range of application are listed above in brackets. In general they are used once you have build a clean base mesh from your 3d scan (pointclouds or mesh) and need to create accurate Nurbs surfaces from it. You might just google the PlugIn pages online to find out about the desired operation area. I think RhinoReverse works best if you need fully freeform like shapes, whereas Mesh2Surface is more suited for mechanical shapes and forms.
Of course Rhino offers to create your own funky tools, for all application ranges.