Depending on your skills and educational background there are many great options for earning extra cash online. People with a background in teaching might be interested in working for websites such as YourTutor, Maths Online or Oz Tutor. If you have a TESOL qualification there are also opportunities to tutor English language students around the world.
If you're a writer, many websites also pay for good quality content, including Matador, Boots'n'all, iworkwell.com and Viator. There are many other websites as well - google 'websites that pay freelance writers' to learn more. There is also a website called freelancer.com on which writers can bid for jobs, although I've never tried this.
Writing for Weekend Notes is also a good source of extra income. However, since you're paid according to the number of people who read your articles it's a good idea to choose topics that you're certain will be popular over an extended period.
If you're just looking for a bit of extra pocket money, I've also been told that Mechanical Turk and Micro Workers are a good way to earn a few extra dollars. Jobs offered by these sites are short and pretty inane (for example, clicking onto websites and 'liking' You Tube videos - presumably to boost SEO results) but the cents earned all mount up. It's a good way for computer-savy teenagers to make some extra pocket money or to utilise spare time when you're waiting for a train or bus!
I am not aware of any and I would be wondering what kind of thing you could do and how it works. It seems that it would be easy to get scammed doing this and end up not paid. I await answers from the informed . . . . .