Blender, Autodesk Fusion 360, Sketchfab, ZBrush, Plasticity, Autodesk Mudbox, 3D-Coat, picoCAD, TrueSculpt, MeshLab, PlastiSketch, Pointshop3D, Acropora, ShapeShop, Vectary, SculptGL, Sculptfab, Bforartists, iDough, PloppSL, Sharp Construct, CB Model Pro, Blacksmith3D-Morph are some alternatives to Meshmixer.