What are examples of an allusion in To Kill a Mockingbird?

There are many many allusions in To Kill a Mockingbird. In chapter 2, we have an allusion to Bullfinch's Mythology, a famous collection of Ancient Greek myths. Scout is explaining to her teacher, Miss Caroline, how it is that she's already able to read. She tells Miss Caroline that Jem thinks that Scout is a Bullfinch not a Finch, and that she was switched at birth. Jem's only kidding, of course, but in his little joke he's alluding to the popular book.
There are also several cultural references throughout the novel. While these are not the same as allusions, these references are important to situate the novel in its historical context. In chapter 2, Jem refers to the Dewey Decimal System, which is used to organize books in libraries. Contrary to what Jem thinks, it has nothing to do with John Dewey, the American philosopher of progressive education.
There is another reference, in the same chapter, when Scout discusses her reading habits and how she never really learned to read; it was just something that came to her. She'd read pretty much anything that Atticus happened to be reading when she curled up on his lap, from Bills to Be Enacted into Laws to the diaries of Lorenzo Dow, a famous Methodist preacher who traveled the length and breadth of the United States.