Free tools for rig weight painting 3D models include Blender (native), Krita (with 3D support), and GIMP (via plugins).
Blender is the most popular—its built-in weight painting tools integrate seamlessly with its rigging system, giving precise control over how weights are applied. Krita uses 3D layers to let you paint weights directly on model previews, simplifying the workflow. GIMP works with 3D assets via plugins like G’MIC, though it’s better for basic weight edits.
If you’re new, start with Blender—its default tools and abundant tutorials make learning weight painting basics straightforward.