As a token of appreciation, the hotel provides complimentary breakfast to all guests.

complimentary 是形容词，由名词 compliment（称赞、赞美）加上后缀 -ary 构成，本义是"表示赞美的"。在商业用语中引申为"免费赠送的"，因为免费服务通常是为了表达善意或感谢。