I've been a member of NCP (National Consumer Panel) for a couple of years now. You can register here and they will contact you if they want you on their panel. They don't pay in cash, but you earn points by scanning your groceries and submitting your transmissions weekly, and by filling out surveys. They provide you with an online catalog and you can choose to buy things with your points. You get points every week you submit your groceries, and when you complete a survey. And sometimes you get bonus points for things!
How do you scan groceries and purchases? They provide you with a little hand hand wireless scanner (click the link above and watch the video - you can see the scanner) and you just scan your purchases and submit through your computer once a week. Even if you haven't purchased anything that week, you can submit a blank transmission and still get your points!
So go ahead and register here - it took me years to get on their panel!