Many veterinarians give away free kittens at some point. The United States has a serious cat overpopulation issue, so many cats eventually end up at veterinarians' offices.
Cats have more kittens in the spring, so a good time to call and check with veterinarians is in the spring and summer. Even if the cat doesn't cost anything initially, it shouldn't be thought of as free. The cat needs many things, such as regular veterinary care, food and vaccines. Sometimes there is a small fee involved in the initial adoption if the kitten has already had veterinary care, such as de-worming or neutering.