Quad Remesher is a industry-standard auto-retopology add-on for Blender developed by Exoside. It uses the same high-performance algorithm as ZBrush's ZRemesher to convert complex, messy geometry into clean, four-sided (quad) meshes with a single click. Availability and "Free" Download
You might think, "It's just a Python script. What's the worst that could happen?" The answer is frightening. Quad Remesher Blender Addon UPD Free Download