Say you buy a dozen doughnuts and the baker throws an extra cruller in your box. You've just been given a lagniappe, or a small gift from a merchant to a customer.
The word lagniappe tells a fascinating American story. Spanish speakers in the New World took a word from the indigenous Quechua language to make la ñapa, meaning "the gift." Then in New Orleans, where Spanish and French mixed freely, la ñapa got Frenchified into lagniappe. In Louisiana it's still used to refer to a bonus from a friendly merchant when you buy something. You could also call any nice little extra a lagniappe.