Power Bash-

This perk doesn’t require a shield.  Bashing is a valuable ability when you find yourself forced to block, and will often stagger an opponent out of a sequence of attacks.  Power Bash lets you Power Attack with a bash, and the resulting smack actually takes most enemies close to your size clean off of their feet.  If you really need to use your Bash to get breathing room rather than just to break up an attack and counterattack, this perk is ludicrously useful- making it very strong for archers and shield-and-spell mages.  In a pinch, it can also be used to send enemies into traps or hurtling to their doom off of high places, though both of those possibilities are terrain-dependent.

Quick Reflexes-

This perk doesn’t need a shield, and praise Talos for that.  If you are holding up a block and an enemy starts a power attack that would strike your block, this perk activates and slows time for you, not only alerting you that you’ve got a block-breaking strike incoming, but also giving you a chance to dodge or break up the power attack buildup with a bash.  If you’re -really- on the ball and have quick reflexes outside of the game, you might even be able to pull off a Power Bash hit and really turn things around.

Deadly Bash-

Another Block perk that doesn’t need a shield to be useful, Deadly Bash multiplies the damage your bashes deal by five- enough to make the damage actually matter!  You’re not going to be smacking anyone to death with just your shield or bow, but having your bash damage actually matter can tip a close battle much more securely in your favor- especially important if you’re a shield-and-spell mage, wear light armor, or want to reduce your Health investment in favor of Stamina or Magicka.

Elemental Protection-

Only useful to people who use an actual shield, Elemental Protection makes fighting a mage or dragon immensely easier.  The majority of spell and breath weapon damage is fire, frost, or shock damage, and cutting it in half by blocking will save your rump.  Combinding it with Spell Resistance and Atronach on a shield-and-spell mage can result in some pretty hilarious wizard fights, cutting incoming spell damage up to 80% (more for a Breton) and refilling on Magicka all the while.  For dragon-fighting, this may mean you want to swap weapons every time it starts belching breath weapons at you when it flies (which can be a pain in the butt but worth the effort), but is particularly useful when your enemy lands and you want your shield to help you against -every- attack.