EdX http://edx.org includes MIT, home of one of the original open educational initiatives -
Open Courseware. http://ocw.mit.edu/index.htm First courses in 2002 and 2003. It is great to see Professor Gilbert Strang http://ocw.mit.edu/index.htm is in the fray. In a few lectures on linear algebra (many years ago) I learned there were actually THREE ways to solve them. Strang is amazing.
Udemy has some free courses, you just have to look for them (check the "free" box):
Stackskills seems to be mostly paid courses. https://stackskills.com/courses.
If you are part of a school or organization definitely check to see if they have already paid for an organizational subscription to any of the paid services.
