The Gadsden flag is a historical American flag, named after patriot Colonel Christopher Gadsden of South Carolina. The association of the rattlesnake and the American colonies can be traced back to publications of Benjamin Franklin in 1751 and 1754. As the American war for independence from Britain grew closer, the snake began to see use as a symbol of Colonial unity and fierce resistance to British oppression (which was fairly mild by today’s standards). The Gadsden flag has become an important symbol to many libertarians. More information on the flag is available here and here.