Free tools for render modifiers on 3D models include Blender (open-source, all-in-one), Daz Studio (free with plugins), and MeshLab (mesh-focused).
Blender has built-in modifiers like Subsurf (smoothing) or Bevel—great for prepping models without extra downloads. Daz Studio uses free add-ons to tweak textures/lighting for better renders. MeshLab simplifies or cleans meshes (e.g., reducing polygons) to optimize before rendering.
If you’re new, start with Blender—it’s intuitive and includes all the tools you need to practice modifiers quickly.