Category Theory

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

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

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

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

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

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

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/

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

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

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

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

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

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

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

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