There are a number of fracture tools available through the Unity Asset Store, which can be used for the creation of destructible objects within the Unity game engine. Some of these tools dynamically fracture assets within the engine, using complex algorithms, (which can sometimes be quite expensive in game). Other tools pre-fracture assets, as is currently still the norm with creating destructibles for a game. Some of these tools include: 

• Ultimate Fracturing and Destruction by Ultimate Game Tools  
  

Alex Dunn is the mastermind behind Low Level Tech and the Fracture tool. 
A playable demo of the Fracture tool in action can be found here.

Havok, (noo... not the X-men guy), is a company who has produced its own vfx software solution, which can be integrated into any film or game development pipeline. 
Havok Destruction is a toolset in this suite that allows for the creation, and simulation, of destructible objects for games. You can read more about Havok Destruction here.