Often, these are tax free if you make under $600 per year, and they are all counted separately. If you do studies for several companies, you can really make some money!
Some studies are purely opinion surveys taken with a target group, and some actually have you use a product for a couple of weeks and then fill out a grading sheet on what you used.
With the former, I have done everything from grocery store arrangements to television news shows.
With the latter, I have tried Swiffer mops, toilet paper, candles, dog/cat food, and very often Pampers diapers. On top of the payment, you get to use the product, which saves you from having to purchase it for at least a couple of weeks.
Of course, this is not a steady income, but instead acts as a great 'perk' to your usual intake. Landing a $55 study could mean not having to pay for prescriptions that month, or an extra treat at the grocer's that week. Landing a $150-$300 study could mean a home improvement, or another chip away at whatever debt you may have.
Getting started on these isn't hard. All you have to do is look for your monthly saver magazine. Around me, it's called Reach. Somewhere toward the back, there is a study opportunity page, usually. Even if the one in question doesn't apply to you, call anyway. Most places often run several different topics at once, and if you're in their database they could give you a call if something that fits you comes up.
I do studies for J&R Coordinating, Various Views Research, Q-Fact, Proctor and Gamble Diapers, and Hilltop Research. The latter has several nationwide offices.
Give it a try... If nothing else, it can be fun to sound off your opinions!