The Virtual Homeschool Group offers online courses for homeschoolers for free. (Students still need to purchase course textbooks for most courses.)
Homeschooling parents who were teaching in homeschool co-ops joined forces to create Virtual Homeschool Group site. They provide and maintain the platform, relying on donations for funding. Courses are created and taught by volunteers, so courses vary in design and teaching style.
They offer a variety of types of courses. Some are actual group courses where students and teachers connect in virtual classrooms, while "at-your-own-pace" courses are designed for independent study by an individual student with open enrollment at any time. Some courses might even be used for your own group class. Courses appear to target about fifth grade and up with many for high school level.
Looking through their class archive, it appears that they began with a heavy emphasis on science, particularly courses based on Apologia science textbooks. They have expanded so that there are now courses in math and language arts. There's even a Speech and Debate course! There are resources for study and thinking skills. However, there remain more courses for science than for other areas.
A new history class (fifth grade level) and a Digital Photography course appear to be in the works with this year's "live" courses (that already have full enrollment) likely being recorded to create "at-your-own-pace" courses for next year. Likewise, live courses in Spanish and Japanese that are full at present might be available at a later date. You can sign up on their list for notification when new courses are made available for enrollment.
Some courses are more sophisticated than others. For example, among the "at-your-own-pace" math courses presently available, the Saxon Algebra 1 course includes recorded lectures, computer-scored problem sets, and computer-scored weekly tests. However, the Saxon Advanced Mathematics course has recorded lectures, but no computerized problem sets with scoring. They do say that "students can scan problems sets and upload for volunteer grading"—an amazingly generous offer for a free course to make.
These courses might well be equal to others that cost hundreds of dollars, so this is really a wonderful option made possible by technology and the generosity of those creating and providing the courses.