Harvard Business School hosts some very interesting psychology and social studies. Payments are range from $10 to $30. For me, I receive their “alert” emails telling me there are study available at their site (some people prefer to check their website themselves), and I login and sign up for studies that are hosted online – most of their studies are hosted on campus though.
I have done several studies/surveys for them, and I highly enjoyed the experience – they are interesting and fun to do. Usually they pay cash (money order), but recently I was paid $20 Amazon Gift certificate for a survey.