I'm an ecommerce consultant, and have worked with 200+ online stores. Everything from micro-niche, single-owner sites to international mega-brands.
They ALL use discounts like this. Sometimes they'll have an app or something that generates one-time use codes that are hard to decipher, but, they probably use that in conjunction with normal codes too.
If you're about to checkout somewhere, go ahead and try a few common ones. Works weirdly often.
Various %s off:
10OFF 20OFF 25OFF 50OFF
Free shipping:
FREESHIP FREESHIPPING SHIPSFREE SHIP4FREE
Buy one, get one:
BOGO BOGO50
Upcoming holidays:
VDAY VDAY2016 HAPPYVDAY VALENTINES VDAY10 VDAY20 CUPID10 CUPID25
Friends & family discounts:
FRIENDSANDFAMILY FRIENDSFAMILY FANDF
Meant to give shit away to important people who will write about it:
BLOGGER AUTHOR GUESTPOST TRIAL DEMO SAMPLE MARKETING MEDIA
Social media (usually limited to 10% off, or free shipping):
FACEBOOK FB10 INSTAGRAM IG10 SOCIAL REDDIT10
They tested coupons when setting up the shop. They don't always delete the code, though:
TEST TESTING TESTING123 DONOTUSE
Look up any recent events they had. They might have codes exclusive for it:
FASHIONSHOW NYFW16
If they have a mascot, or the owner has a dog/cat featured on the site, there will almost always be a discount code related to it.
WALLY FIDO WOOF WOOFWOOF MEOW
They are always case-sensitive, and almost always uppercase. If you really need a code, go look up some other ones they had in the past. If they've used Xmas2013, Xmas2014, and Xmas2015 ... you have a pretty good chance of using Vday2016.