Coursera offers an extremely diverse selection of courses from prestigious universities all around the world including the University of Michigan, Johns Hopkins University, Princeton, and Duke University, among many others. Each course is made up of thousands of students enrolled all around the world and offer video lectures, graded quizzes, activities, and discussion forums.
These courses may not offer college credits or go toward a graduate program, but they do offer an opportunity to learn about some topics you're not familiar with. For example, I'm currently enrolled in Astrobiology and the Search for Extraterrestrial Life. The nerd side of me tries to catch every episode of every show about the universe on the Science channel, so this was a perfect supplement to that. I like to be well-rounded, and I think this is a great way to do that, only dedicating 3–6 hours per week, per course.
I'm also enrolled in a class titled An Introduction to the US Food System: Perspectives from Public Health. Only two weeks in, and I've learned about the threats we face with both the environment and the cost of production of livestock, as well as the growing population of the world and our inability to meet that demand.
Finally, I'm enrolled in Human Evolution: Past and Future. I have no background in anthropology or the history of human evolution. But I do love science, as you can probably tell, and find it fascinating to learn and to see the world through the eyes of another field and especially through the eyes of experts in those fields.
Four $49 you can receive a Certificate of Accomplishment through Coursera showing you did finish the course which includes the title of the course, the university through which the course was offered, and a signature from the professor. Coursera also offers specializations in which you can earn a Specialization Certificate. A specialization consists of a series of courses in which you complete and pay for the $49 Certificate of Accomplishment for each course and finish with an all-encompassing Capstone Exam or Capstone Project.
Whether you'd simply like to deepen your knowledge or expertise in an area, or are looking for a free way to invest in yourself, Coursera is a valuable tool to improve yourself. Utilize it!