c'est cadeau/c'est un cadeau

Meaning: it's free, it's on the house
Literal translation: it's gift

Notes: The French expression c'est cadeau means that something is a gift, but not a personal one.

Rather, it's something that a store or restaurant is giving you - what is often redundantly called "a free gift." Note the lack of an article: c'est cadeau and c'est un cadeau have slightly different meanings.

C'est cadeau means that a business is giving you something for free, whereas c'est un cadeau means that someone you (probably) know - a friend, a family member - is giving you a gift. Examples:

-Achetez deux croissants et le café, c'est cadeau !
- Buy two croissants and the coffee's on the house!