Just wondering; can normal enemies drop Legendary quality equipment? What is the breakdown for probability for White/Blue/Yellow/Purple?
Normal enemies CAN drop a Legendary. Breaking barrels can drop a legendary. There is a chance of it happening, albeit a small one.
NOTE: As of patch 1.0.3, the chance for loot drops from containers is even smaller.
As you have probably noticed, the Elite and Unique enemies tend to have a better chance of dropping better loot. You are more likely to get something good off these mobs, but that doesn't mean a normal enemy couldn't give you a great and rare weapon.
As far as the probability or formula, no one really knows the answer except an official source that would never disclose it.