I have my name in with Fieldworks. I don't know if they're national -- they have several offices in the Chicago area. I have done several focus group sessions with them (one on Dunkin' Donuts new logo, Kraft parmesan cheese, long-term care insurance, computer game membership sites, and a few more). All have paid at least $75 cash on completion of the study. My two sons have done some as well -- my DS 13 did one on a video game for Kung Fu Panda and one on cheese cracker snacks; the older son did one on Bionicles and was "backup" for another study (he and his friends waited in case the whole group already scheduled didn't show up -- my son's group didn't do the study, but still got paid). These groups are legitimate, don't know how you'd find them though! However, in general, this is not a way to make a lot of money -- I've been called for way more focus groups than I've qualified for, and they don't like you to do more than 1 in 6 months. They're fun and I like the extra money, but it's certainly not reliable.
If you have a personal care products company near you (like Proctor & Gamble), they may have a consumer products testing arm. There used to be (don't konw if there still is) a company around here that will provide you with about-to-come-out products like shampoos to try out and give your opinions on. Those were pretty regular.
Try googling "consumer focus group studies" or something like that. You may come up with something!
Erin