I've been using Wikibuy along with Honey for some time now, and while both are very useful, frequently Wikibuy comes up with the lowest price.

it's not perfect, like many such tools it can get the product search wrong and return a low price for a different product or size, but it doesn't happen very frequently, and it is easy to report such errors.

I'd hate to be stuck with just one shopping add-on, but this is one of my top candidates.

Finally available on Firefox. Automatically applies coupons on favorite sites and tells you when there are better prices on products you're looking at.

This saves me so much money when shopping online.