Receipt Hog is a free App that you can get from iTunes for your iPhone. It is currently not available for Android. The App essentially pays you for sending in photos of your grocery receipts which are used for market research. Receipts are sent in anonymously.
As you upload receipts you get points for each one. It does not matter how much you spend, the points are the same. As you upload more receipts the points increase from 10 per receipt in the beginning to up to 25 per receipt. If you are one who shops in frequent small trips this App could really pay off for you.