Shields are extremely handy.  They give bonuses to AC, but they are also breakable.

Any time you are hit in combat, you can sacrifice your shield (it is splintered and useless) to negate the damage of the attack.  This includes critical hits.

You may also use this effect if you are stuck by a direct-damage spell or area-effect spell.

If you have a magical shield, the shield may make a save (based on the material of the shield) with a modifier equal to it's plusses to avoid destruction.