Pierre de Fermat was a French lawyer and mathematician who may be thanked for major developments in geometry and calculus. He also wrote his last theorem in the margins of a paper. In the spirit of all the rich things scribbled in margins (paper and digital), Fermat's Library is a space where "professional scientists, academics and citizen scientists" can annotate one another's academic papers, mathematical equations, and more. Since 2015, the team behind Fermat's Library has provided a free open space for such collaborative annotation. To add comments to any paper, users simply select the paper and click on the portion of the text where they would like to comment, and type a note. As of this write-up, visitors can explore Alan Turing's "Intelligent Machinery, A Heretical Theory," Paul Krugman's "A Theory of Intelligent Trade," and the United States Government's Immigration Act of 1924. Readers are also encouraged to suggest their own papers for annotation by the community.