- A Gentle Introduction to Category Theory - Lecture notes by Maarten M. Fokkinga introducing some important notions from category theory, in particular adjunctions. Proofs are given in a calculational style, and the (few) examples are taken from algorithmics. The text is a long PostScript file.
- Categorical Myths and Legends - An archive of stories about category theorists.
- Categories Home Page - Web page for the category theory mailing list.
- Categories, Quantization, and Much More - Introductory article by John Baez.
- Category Theory - This expository article is an entry in the Stanford Encyclopedia of Philosophy.
- Category Theory and Homological Algebra - In the "known maths" series.
- Computational Category Theory - An implementation of concepts and constructions from category theory in the functional programming language Standard ML. Documentation and code.
- CT Category Theory - Section of the e-print arXiv dealing with category theory, including such topics as: enriched categories, topoi, abelian categories, monoidal categories, homological algebra.
- Descent and Category Theory Connections - Maintained by M. Alsani.
- Groupoid Home Page - Maintained by Birant Ramazan. Address book, open problems, meetings, pictures, other resources.
- Groupoids - Notes by Ronald Brown.
- Higher-Dimensional Categories - An illustrated guide book by Eugenia Cheng and Aaron Lauda (PS/PDF).
- Open Problems on Model Categories - Problems on model categories listed by Mark Hovey at Wesleyan University.
- Paul Taylor's Home Page - Includes papers on category theory.
- Structures Directory - Email directory of logicians, algebraists, and programming linguists working primarily on structural problems in mathematics and computer science.
- The Computational Category Theory Project - The aim of the project is the development of software on a wide variety of platforms for computing with mathematical categories and associated algebraic structures.
- Toposes, Triples and Theories - By Michael Barr and Charles Wells, 1983. A revised and corrected version is now available free for downloading. Formats: DVI, PDF, PostScript.