This has got to easily be one of the best Chrome extensions for anyone who shops online, and most of us do.
Honey is an extension which searches for coupon codes for you. So next time you go to buy something at a major retailer like Amazon, you'll be presented with a new button named "Find Savings", you click on that button and it'll search for coupon codes. It works on over 100 stores across the US, Canada and the UK. It has a huge list on the extension page, showing which sites it covers.
They're also adding a ton more sites every day. It's an absolutely amazing way to save money quickly when buying goods online.