Massive Open Online Courses are the cutting edge of adult education. You can take classes from some of the best universities in the world. Some of the MOOCs will offer certificates for a fee, but the courses themselves are free. Most involve a video component, reading, and quizzes.
The largest and best-known platform is coursera.org. With partners such as the University of Michigan, Stamford, UPENN, Duke, Rice, and 1,800+ courses ranging from architecture to zoology, Coursera will lay out an educational plan for a wide range of degrees.
Another big name is edx.org. They partner with Harvard, MIT, the Sorbonne to name a few and have 1700+ courses.