Arquest is a company that conducts diaper studies by allowing parents like yourself to test them on your children. I have done one of these studies myself, so I know for sure that it's legitimate. They send you a pack of diapers in the mail, you use them and take a short survey about them, and then they send you a $10 check!
If you're interested, you can call these numbers and request to be contacted when they have a study available.
1-888-342-7372 ext. 646 (Girls)
1-888-342-7372 ext. 634 (Boys)