Unschooling, as a form of homeschooling, is legal in all 50 states. Each state has its own regulations about unschooling/homeschooling and what they expect home educating families to do. Some states require testing; some don't. Some require a lot of paperwork; some don't. Many states require no contact at all.
Internationally, unschooling is also legal in a variety of countries (including Australia and Canada). But many countries have laws prohibiting home education in general or have strict restrictions.
Find a homeschooling group in your state or country where people who have been navigating the regulations for years will be most helpful. They will have more specific details for you.
Ann has an enormous collection of information about the legalities that home educators face as well as a list of state by state description of the laws that pertain to homeschooling.
Countries with the most prevalent home education movements include Australia, Canada, New Zealand, the United Kingdom, and the United States. Some countries have highly regulated home education programs as an extension of the compulsory school system; others outlaw it or make socially make it virtually non-existent in their country.
Find out what's required in your state or country. Ask the local unschoolers. We have a collection of links to unschooling groups all over the world.