Paul Taylor's Home Page
- Includes papers on category theory.
- http://www.cs.man.ac.uk/~pt/

Groupoids
- Notes by Ronald Brown.
- http://www.bangor.ac.uk/~mas010/gpds.htm

Groupoid Home Page
- Maintained by Birant Ramazan. Address book, open problems, meetings, pictures, other resources.
- http://unr.edu/homepage/ramazan/groupoid/

Higher-Dimensional Categories
- An illustrated guide book by Eugenia Cheng and Aaron Lauda (PS/PDF).
- http://www.dpmms.cam.ac.uk/~elgc2/guidebook/

Computational Category Theory
- An implementation of concepts and constructions from category theory in the functional programming language Standard ML. Documentation and code.
- http://www.cs.man.ac.uk/~david/categories/

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.
- http://front.math.ucdavis.edu/math.CT

Categorical Myths and Legends
- An archive of stories about category theorists.
- http://www.mcs.le.ac.uk/~ah83/cat-myths/

Categories Home Page
- Web page for the category theory mailing list.
- http://www.mta.ca/~cat-dist/

Open Problems on Model Categories
- Problems on model categories listed by Mark Hovey at Wesleyan University.
- http://claude.math.wesleyan.edu/~mhovey/problems/model.html

Descent and Category Theory Connections
- Maintained by M. Alsani.
- http://north.ecc.edu/alsani/descent.html

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.
- http://www.cwru.edu/artsci/math/wells/pub/ttt.html

Structures Directory
- Email directory of logicians, algebraists, and programming linguists working primarily on structural problems in mathematics and computer science.
- http://boole.stanford.edu/pub/structdir

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.
- http://wwwhome.cs.utwente.nl/~fokkinga/mmf92b.html

Category Theory and Homological Algebra
- In the "known maths" series.
- http://www.math.niu.edu/~rusin/known-math/index/18-XX.html

Categories, Quantization, and Much More
- Introductory article by John Baez.
- http://math.ucr.edu/home/baez/categories.html

Category Theory
- This expository article is an entry in the Stanford Encyclopedia of Philosophy.
- http://plato.stanford.edu/entries/category-theory/