Softwrap - Dynamics For Retopology approaches retopology differently from most add-ons. Instead of building new topology from scratch, it reuses the topology of similar or previous models, which is faster than starting over. A free human basemesh made by the developer is available to use as a starting point, even without purchasing the product.
The add-on runs a custom softbody simulation while snapping in a way similar to the shrinkwrap modifier, which is where the name comes from. Its simulation engine is non-physically-based and built specifically for retopology, with custom dynamics that let you freely manipulate the mesh while it automatically snaps onto a sculpt or scan. A set of sliders controls the behaviour.
Version 2.0 rewrote the code from scratch with a new simulation engine that produces smoother, more flexible results. Snapping supports inside, outside and surface modes, and options include a gravity force, a Seam Force similar to Blender's cloth simulator, Quad Smooth and Topologic Smooth settings, and a Simulation Group to lock vertices from a vertex group. Softwrap is compatible with Windows and Ubuntu; proper Mac support isn't offered.