Oh, how the internet changes things. It wasn't too long ago when people laughed at the idea of shopping online. Now we have online courses to strengthen our arsenal. It's always a good idea to learn something new, and nowadays there should be fewer and fewer excuses why you can't learn something new.
Khan Academy
Great site for math-oriented fields. I've used this site a lot throughout the years. Whether you want to learn something new or just brush up on your skills; it's worthwhile. It's not just for students though. Teachers and parents can also make accounts to learn how to better help and understand formulas and such for themselves.
Math (especially higher levels)seems to be one of those areas of study that need to be reviewed again and again until it sticks. This can often be discouraging in the real world when a student wouldn't want to ask the same question five times in a row, trying to clarify. Now, a student can rewatch a video a hundred times until they get the concept, with no risk of looking foolish in front of peers.
Coursera
This website has more than just math courses. Courses ranging from equine nutrition to learning how to learn. There is no shortage of courses. One of the coolest things about this website is where the courses are from. Many of these places the average person would never attend; like Stanford, Berkley, or some Univesity off in China. You're exposed to courses and ideas that you usually wouldn't have the opportunity to.
EDX
www.edx.org
I just stumbled across this site today. Looks very similar to Coursera. I haven't been through all the courses, but I imagine there are some different ones. Again, you have access to material from Harvard, MIT, and other academic institutions.
Doulingo
This is one of my favourite sites. I've used this a bunch. It's my dream to be a fluent polyglot someday, and this has helped quite a bit. The site is very gamified. As you complete challenges you are awarded a gold seal for that small category. As time marches forward the gold seal will have bars that will start to drain. You have to continue to use the site or eventually everything will drain. The idea behind it is to compare it to your brain. As time goes by, you start to forget what you've learned, the website tries to reflect that.