The Nielsen Homescan Consumer Panel is accepting new applicants, and they don't do this often so if you are interested, sign up fast! I joined in the summer and so far this program has been pretty easy.
If you are accepted with to the panel, you will be sent a small hand-held scanner (mine pictured above). Then you scan the UPC on everything you purchase, or use a card they provide for things like eating out to buying gas. The scanner goes through your internet connection to transmits your data, then you earn points which you can redeem for prizes. Simple huh?