Open online classes can broaden your knowledge, but don't pin your career hopes on them yet.

1. MOOCs are huge. Massive open online courses (or MOOCs) are college-level courses available to anyone. Lectures and course materials are accessed online, and tests may be computer graded or peer reviewed. Started as experiments to connect students from all over the world, the numbers didn't hit truly "massive" proportions until 2011, when Stanford professor Sebastian Thrun offered Introduction to Artificial Intelligence and 160,000 students from 190 countries enrolled.

2. How to focus in. Three big companies provide the majority of MOOCs. Thrun launched Udacity, which was soon followed by Coursera, also founded by Stanford professors. (Princeton and Yale are among the Ivies whose courses are listed on Coursera.) The third is edX, a nonprofit joint venture of Harvard and MIT. You can search listings for all the major providers at http://www.class-central.com.

3. Satisfaction, yes. Credit, no. The most you can hope for at this point when you take a MOOC is a certificate of completion. All three major providers charge for a verified certificate that assures your tests were digitally proctored or your identity and coursework have otherwise been checked. Last year, the American Council on Education endorsed five MOOCs for credit (two from Duke University, two from the University of California-Irvine and one from the University of Pennsylvania), which may signal greater acceptance.

4. Reach for the stars. Lifelong learners can take introductory courses in everything from astronomy to physics. Looking to enhance your skill set? Courses in computer science and engineering are plentiful at Udacity. Or maybe you just want to follow your passion. For baseball junkies, Boston University recently offered Sabermetrics 101: Introduction to Baseball Analytics. Beatlemaniacs could take The Music of the Beatles, from the University of Rochester.

5. Test-drive a new career. Use a MOOC to see if you like the field. But to gain clout with an employer, you'll need a certificate program or master's degree. Georgia Tech is offering the first MOOC-based master's in computer science. Anyone may take the classes, but if you want to get on the degree track, you'll have to be accepted into the program, take in-person proctored exams and pay about $7,000 over three years.

6. Earn digital badges. Michael Nanfito, executive director of the National Institute for Technology in Liberal Education, says that in the future, "online learning, including MOOCs, will get more formal acknowledgment of its value." For example, Secretary of Education Arne Duncan supports a system of "digital badges" -- which could include MOOCs -- to allow students to showcase skills not reflected in their diploma.

(Jessica Anderson is an associate writer at Kiplinger's Personal Finance magazine. Send your questions and comments to moneypower@kiplinger.com. And for more on this and similar money topics, visit Kiplinger.com.)

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