I like www.constant-content.com. It is not an immediate payout for the work that you do, but you can build an inventory in their system (they will actually tell you which categories have a need for more content, so really is not hard to figure out where you should focus your attention). The reason I like this platform is because once you have built up a good "catalog" of articles in their system you will start to build yourself a bit of a legacy income that is largely passive.
I have also had a lot of success in the past with ifreelance.com although I am not a fan of bidding for projects), and http://www.textbroker.com no bidding on projects, just select the one you want and start on it).
Hope that helps!