In general, free online classes are only suitable for continuing education, not for getting college credit. However, the American Council on Education has approved some free online courses for college credit.
Colleges sometimes offer online classes for free. Professors at the college often teach these courses. It may even be possible to take an exam and earn a certificate for having passed the course. However, these certificates are not transferable for credit. They are only suitable for adding to a resume as continuing education.
Five free online courses through Coursera have been approved for college credit: intermediate algebra, bioelectricity, calculus, pre-calculus and introduction to genetics and evolution. As of 2015, a sixth course has been added — Kaplan University Open Learning's documenting your experiences for college credit.