I think this topic came up in a more recent thread. I use Coin Out, Fetch, Receipt Pal, Receipt Hog, Receipt Jar, Ibotta, Rakuten (formerly ebates), and Mr. Rebates. The last two are not receipt apps, but you do get cash back from shopping online on certain websites (e.g. Lowe's, Home Depot, etc.). A lot of times for those I'll order online and pick-up in store.
The key is to game the system. If you only use one app, probably not really worth the time. Combined, however, you can make some decent money over the long-haul. I am getting ready to cash out of everything. When I do, I'll have over $500 in cash and/or gift cards, depending on the app or what I choose. I do what @chrismcmillon does and scan while watching TV. I normally scan once per week. If I am really busy with shops or other things, I may miss. And that's okay. This is just some extra money that is easy to earn. You can scan the same receipt across multiple apps. You can also connect your email and store accounts to get credit for online purchases. Some people are not comfortable with this, but I figure they are watching anyway, so might as well get something in return for it.
There is the truth.
Then there is the right thing to say.