This whole novel is about Mockingbirds who do nothing but sing so we can enjoy their song and never do any harm. Certainly I agree with the ones listed, but I would say Atticus himself is one of the most obvious examples of a mockingbird. He suffers the prejudice and insults of the bigots in town and nearly gets caught between a lynch mob and Tom Robinson, but all he's trying to do is to make the world better by bringing truth and justice into it.
And I'm definitely one that sees the Mayella as a mockingbird all the way up to the point where she goes along with her father's rape accusation against Tom. She's most likely sexually abused, and yet she tries to care for her younger brothers and sisters and raise those dang flowers of hers. But faced with the town's hatred and racism and given a choice between admitting that she initiated sex with a black man or making a false accusation, she gave up her mockingbird status by striking out with the viciousness of a vulture.