The past few years have seen a boom in online learning. Nearly one-third of all students in higher education (6.1 million) are taking at least one online course, according to the Babson Survey Research Group, and that number is projected to grow to more than 25 million by 2015, according to research firm Ambient Insight. Thanks to the efforts of institutions such as MIT and Stanford, it is becoming easier than ever for anyone with a computer and access to the Internet to sign up for courses taught by some of the world's foremost professors.

All of the below online learning platforms offer open online courses or Massive Open Online Courses ("MOOCs"), which often see hundred of thousands of people enroll in a single course. The free course initiative at MIT, for example, has been used by more than 100 million people worldwide since it debuted a decade ago, and it aims to reach 1 billion minds in the next decade. Each platform is different -- most are free, some are paid; some offer credit, some offer those who complete a course a certificate of mastery and a grade, but no official credit; some feature hour-long lectures, and others, segments as short as 10 minutes; some are self-paced, others hold class at the same time each week. Check out the incredible resources below to find the best fit for your interests, personality and schedule.