A list of Illinoisan companies. Many well known Illinois companies have been included. While this list isn't definitive, these are most of the top businesses in the state of Illinois. As a country, the United States has encouraged entrepreneurship throughout much of its history. The state of Illinois is no exception to this rule, and new businesses are started in Illinois daily, ranging from corporations to LLCs to partnerships to sole proprietorships. This list, however, is mostly focused on the largest corporations in the state of Illinois.