ResearchMatch is a site that helps researchers pair up with Participants for research Studies. simply fill out the registration and wait for a researcher to contact you for a potential study. Anyone living in the US can sign up--all ages and backgrounds are welcome.
although most of the studies seem medical based you do not need to have a medical condition to be selected for studies. in fact, i was just contacted for my first study through them and the only prerequisite was being a married couple with children.
since this site is only available for non-profit organizations (think university research departments), they may not be the highest paying studies but it is still worth signing up for since you can always decline any study.
here is the link to join. even though they describe participants as 'volunteers' i believe the majority of studies are in fact paying ones--my Current study pays $90.
here is a snapshot ResearchMatch's current stats. nearly 500 current studies!