Online resources:

* [The Catsters channel][1]
* [MATH198 course notes][2] (ongoing)
* [Rydehard, Burstall: Computional Category Theory][13] - uses ML to illustrate concepts.
* [MAGIC course][3]
* [Barr, Wells: Category theory for computing science][4]
* [Jaap van Oosten: basic category theory][5]
* [Tom Leinstner][6]
* [Eugenia Cheng][7]
* [Steve Awodey][8] - very similar to the book mentioned by Quadrescence
* [Daniele Turi][9]

Books:

* "Basic category theory for computer scientists" by Benjamin Pierce
* MacLane - solid mathematical foundations, but hardly any references to computing

Category theory in Haskell:

* [Wikibooks introductory text][14]
* [sigfpe's blog][10] has a lot of category theory articles - (di)natural transformations, monads, Yoneda lemma...
* [Comonad.Reader][11]
* [The Monad.Reader][12] - check "Calculating monads with category theory"

[1]:http://www.youtube.com/user/TheCatsters
[2]:http://haskell.org/haskellwiki/User:Michiexile/MATH198
[3]:http://www.cs.manchester.ac.uk/~hsimmons/MAGIC-CATS/magic-cats.html
[4]:http://www.cwru.edu/artsci/math/wells/pub/ctcs.html
[5]:http://www.itu.dk/~birkedal/teaching/category-theory-Fall-2001/basiccat.ps.gz
[6]:http://www.maths.gla.ac.uk/~tl/ct/
[7]:http://cheng.staff.shef.ac.uk/catnotes/categorynotes-cheng.pdf
[8]:http://www.andrew.cmu.edu/course/80-413-713/notes/
[9]:http://www.dcs.ed.ac.uk/home/dt/CT/
[10]:http://blog.sigfpe.com
[11]:http://comonad.com/reader/
[12]:http://www.haskell.org/sitewiki/images/8/85/TMR-Issue13.pdf
[13]:http://www.cs.manchester.ac.uk/~david/categories/book/book.pdf
[14]:http://en.wikibooks.org/wiki/Haskell/Category_theory