Demand Studios is a great site and I am really enjoying myself over there. They are paying me $15 per article although I have heard it is between $10 and $20 per article.
You must submit an application. It took me a month to hear back from them, but I am glad I did. I got accepted. Now here is how it works. You are allowed to claim 10 assignments at a time. There are tons to choose from in all kinds of subjects. Then you make sure you get them done by the deadline. When they are accepted than you can claim 10 more. If you do this well a few times they will increase you to 20 assignments at a time.
Payday is every Friday through Paypal. That is the only way to get paid so make sure you have a Paypal account. They pay promptly and are a very professional site. You choose the assignments that you feel you can write on. Pay per article is excellent if you ask me.
Here is the link for Demand Studios: http://www.demandstudios.com