Online resources:
- The Catsters channel
- MATH198 course notes - examples in Haskell
- Rydehard, Burstall: Computional Category Theory - examples in ML
- MAGIC course
- Barr, Wells: Category theory for computing science
- Jaap van Oosten: basic category theory
- Tom Leinster
- Eugenia Cheng
- Steve Awodey - very similar to the book mentioned by Quadrescence
- Daniele Turi
- Thomas Streicher
Books:
- "Basic category theory for computer scientists" by Benjamin Pierce
- MacLane - solid mathematical foundations, but hardly any references to computing
- Abstract and concrete categories - might be considered too verbose, but it's full of examples
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"