The “Crack It” add-on makes a cracked object based on a selected object. It can be used to make cracked shapes such as rock, tree bark, or poop, etc..
“Crack It“ addon is included in upcoming Blender 2.79 release, as an official community add-on.
So now you can use the addon without downloading addon files.
Does anyone remember the BlenderNation’s article about “Hhow to make realistic excrement”? Development of the add-on actually started from the technique.