I signed up for Paribus about a month ago, and it took just about that long before it found any savings for me. But it did, and Labor Day weekend alone, it saved on 3 purchases for me without me having to do anything beyond signing up for the service in the first place.
It uses fancy algorithms to find savings available on your purchases. You give them access to your email account so that it can find your purchase receipts and if there any savings available that you didn't receive. You link your credit card, too, but of course, the company uses all kinds of security measures to keep that info safe. Also, you should be checking your account regularly so that if any funny business happens, you can fix it quickly.
So it tracks your purchases and prices for that purchase, and it writes an email for you to the vendor to request the savings. Most times, it'll get refunded to your credit card, but once, I got a gift card from a vendor I use regularly anyway so I'll get use out of it. Paribus keeps 25% of your savings, but hey, a small price to pay for savings I most likely would not have found on my own. And, of course that means if they don't save you any $$, they don't get any $$ either.
I love this kind of lazy, no hassle savings. I am so not good at clipping coupons, plus I value my time a lot more than 25 cents off something. I'm also not good at going to sites like eBates or Groupon to check for lower prices (though on occasion, I remember to check retailmenot.com).
I'm only recommending Paribus because it's so easy to use. Having said that, I think if I get enough referrals, their commission percentage goes down for me.