Online resources:
- The Catsters channel
- MATH198 course notes - examples in Haskell
- Rydehard, Burstall: Computional Category Theory - examples in ML (free reprint of a book)
- MAGIC course
- Barr, Wells: Category theory for computing science (TAC TR22 is a free reprint of the book)
- Jaap van Oosten: basic category theory
- Tom Leinster
- Eugenia Cheng
- Steve Awodey - very similar to the book mentioned by Quadrescence
- Daniele Turi
- Thomas Streicher
- Abstract and concrete categories: the joy of cats - might be considered too verbose, but it's full of examples; slightly newer (?) version as TAC TR17
- Spivak: Category Theory for Scientists - free textbook of a 2013 MIT OpenCourseWare; an updated (and non-free) version was published by MIT Press in 2014.
- Emily Riehl: Category Theory in Context
Books (not free):
- Benjamin Pierce: Basic category theory for computer scientists, MIT Press 1991; a slight expansion/update of the earlier (and free) CMU-CS-88-203 report
- MacLane - solid mathematical foundations, but hardly any references to computing
- Martin Brandenburg - Einführung in die Kategorientheorie (in german)
Category theory in Haskell:
- Wikibooks introductory text
- sigfpe's blog has a lot of category theory articles - (di)natural transformations, monads, Yoneda lemma...
- Comonad.Reader
- The Monad.Reader - check "Calculating monads with category theory"
- Bartosz Milewski - Category Theory for Programmers
