As of 2015, a good location to find free quizzes about Martin Luther King Jr. is the Scholastic website. These quizzes can be taken and scored online, or be printed.
The Scholastic site also offers information on Martin Luther King Jr., pictures and copies of his famous speeches.
Another good source for quizzes about Martin Luther King Jr. is the Teacher Vision website. The quizzes on Teacher Vision are tailored toward students in grades 4-8 and can be printed. The site offers five free printable items, and then users must pay a subscription fee in order to access other material.