Decal Beast (Free Version) is a Blender add-on from Bala for adding graffiti and decals to 3D models. You drag and drop a decal onto any mesh, and it aligns automatically to the surface normals. A visual image gallery shows thumbnail previews, and you can organize decals into category folders for quick access.
Placement is interactive: hover over the mesh, click to place, then drag to rotate and scroll to resize in a two-step workflow. The add-on works on objects with multiple materials, wiring up the nodes so a single decal spans them under one empty. You can also bring in your own images, since any PNG, JPG, or BMP from any folder becomes a decal instantly.
The workflow stays non-destructive. Decals live as node groups on empties, so deleting one leaves the original material untouched, and per-decal controls let you tweak each node group's inputs from the sidebar. A refresh and prune option rescans the scene and can remove decals from untouched materials. It supports Blender 4.5 through 5.0 and works with Cycles, Eevee, Blender-Internal, and the Blender-Game-Engine.