PennyCat delivers coupon codes and discounts to you for various online stores before you check out. Fire up Facebook Messenger (or send it a message on Facebook directly), tell it the store you need a code for and the bot will search for you and spit out a code to try. I tried PennyCat with a few different retailers with varying luck. I searched for NewEgg, for example, and, after clarifying which NewEgg you're shopping at (versus NewEgg Business, Canada, or another branch), and I turned up 856 coupon codes on various items. Search for BestBuy, and I got a bunch of sites not related to the actual electronics retailer, easily fixed by searching its proper name, "Best Buy" (with the space). There's also some support for Australian sites, with a search for Myer turning up four coupons. In any case, your mileage may vary, but when it worked, it worked well. PennyCat won't always spit out a coupon code you can use, however, and sometimes the only codes available are for specific types of purchases or things you're not planning on buying.
That said, it's a fast and simple way to search for discount codes and coupon codes in that second before you check out and wonder if you could save yourself a little money on shipping or shave a bit off the total. The service also has a bunch of other hooks to try and get you to stick around, like "penny cash" or Facebook messenger games you can play, but steer clear of that stuff and stick to its purpose and you'll be fine. Also for the record, PennyCat makes money from referral fees, so when you click through to shop, that's how the service earns its keep. Hit the link below to read more about it, or try it out.