Global Online Academy - this is a certified school offering unique and exciting electives. ISB has agreements to allow students to enroll in these courses (NOTE: costs may be associated with the courses)
edX - Courses and certificates in many academic areas from leading universities from around the world
Harvard Summer School - a smaller offering yet unique offerings for students for summer extension (NOTE: costs may be associated with these courses)
Study.com - has courses in almost every academic area offered by academic institutions including college preparation and selection and more.
Coursera - courses and certificates from both business and academic institutions with an amazing selection of fields of study