Pieces and fragments of various sizes. Rubble looks believable because it was created using physics and shatter simulations. Endlessly useable in war scenes, ruins, etc.
Just throw on a texture and have fun!
OBJ (OBJ, filesize: 3.35 MB), MAX (Autodesk 3ds Max, filesize: 10.9 MB), FBX (Autodesk FBX, filesize: 13.1 MB)