I just come across Paribus by a friend who recommended it to me. The way it works is, it scans your emails and looks for receipts from big online retailer stores like Amazon or Target etc. Then it compares the price you paid with the current price of it. If there has been a price drop on that product since you purchased it, you then get a refund.
It's free to sign up to Paribus, you have to give it access to your emails obviously. Then they look out for price reductions on any thing you have purchased from their selected merchants and retailers. Then when there is a price drop, Paribus will contact the store for you and get a refund for you. Apparently.
If they are successful in getting you a refund, they do keep 25% as a "success fee". But you get to keep the rest obviously including the item you actually purchased. Again, "apparently".
Their merchants include giants like Amazon, BestBuy, Newegg, Staples, Target, Bloomingdales, Gap, Walmart, Macy's, Apple and several other merchants.
And you'll have to check out their terms of service. Although the strange thing is, their TOS page doesn't load for me in Firefox. Maybe it's just a browser bug I don't know but I'd really like to see their terms of service and find out more about how they work.