Nielsen HomeScan is a program that is FREE to join and you are never under any obligation to continue the program. You fill out their application and once accepted are mailed a handheld scanner which you scan all of your purchases into each week.
The information you scanned is uploaded via your computer once a week and that's it! You earn points for each week you scan and upload the items and the points convert to prizes! There are also occasional surveys to complete that give you points. Remember "playing cashier" when you were little? Now you can do it as an adult, from the privacy of your own home - and even earn some prizes! ;)