The kids are back in school, the year is winding down, and maybejust maybeyoure finding yourself with a teeny, tiny bit of extra time on your hands.Instead of plopping down in front of the TV or running your phones battery dry on social media, why not expand your educational horizons?Whether youre looking to learn a new skill, advance your career, or simply satisfy your curiosity, these four platforms offer some of the best free online courses available.CourseraWith north of 5,000 free courses to choose from, Coursera offers one of the largest collections of learning content online.The site partners more than 300 universities and companies to offer educational content across a broad range of disciplines.While certificates require payment, the majority of course content remains freely accessible via the sites audit option.edXFounded by Harvard and MIT, edX hosts more than 4,000 courses, with around 1,000 available to audit for free.The catalog includes everything from introductory courses to advanced professional certificates, though like Coursera, certificates and graded assignments require payment.The course selection covers the full academic spectrum, but skews toward the sciences and engineering.FutureLearnFutureLearn offers limited access to around 1,400 of its courses.The catch? Lessons covered by the freebie plan are doled out on a weekly basis instead of being available all at oncewhich might actually be a nice cadence if youre looking to learn at a more leisurely pace.The course catalog spans an impressive range of subjects, but with particular strength in humanities, healthcare, and technology.Khan AcademyNon-profit Khan Academys commitment to completely free education makes it unique among the major learning platforms.The sites strong focus on STEM subjects covers a wide variety of material for school-aged learners, although theres still plenty here for those whove graduated from backpacks to briefcases.More than 10,000 video lessons are available here, and theyve got a reputation for clarity and concise explanations, making complex topics accessible to learners of all levels.