What are some sites that specifically pay users for online studies, paid focus groups, etc.?

I am not talking about survey sites; rather, sites that pay you for studies or focus groups. I always stumble upon these type of things on survey sites (such as iPoll, Swagbucks, etc.) and have always wondered if there is a site that specializes in this sort of thing. For instance, I was doing an iPoll "survey" to find that it was for a 4 week, online study group where the total payout was to be $300.00USD. . . . I was unfortunately DQ'd.