Toddler throws uncontrollable tantrums

What should we do when our toddler explodes in a fit so violent that it's just about impossible to calm her down? She has always been strong-willed and prone to tantrums, but just recently there have been occasions when she has become so frustrated that her behavior has spiraled completely out of control. At such times she wildly hits everything and everyone within reach, including herself. What do you suggest?