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!
Also...although I mentioned iworkwell.com, I've never tried it, but just heard about it from a friend. I just logged on to it then, and don't think that it's particularly relevant.