Coursera: These classes from a mix of US and international universities have a schedule of lectures and assignments by week and are peer-reviewed; providing three reviews of other students’ work is required. www.coursera.org
Udacity: Udacity offers a mix of free classes and paid, self-paced courses. www.udacity.com
Education Portal: Courses specifically designed to meet exam requirements: GED, AP, CLEP, etc. Self-paced, navigable 10-minute mini lectures that include quizzes; a wrong answer on the quiz delivers you back to the spot in the lecture where the material was covered. http://education-portal.com/