[platz]

I'll second Haskell wiki. It has some of the most detailed explanations of functional concepts I've come across.

http://www.haskell.org/haskellwiki/Fold

http://www.haskell.org/haskellwiki/Monad

http://www.haskell.org/haskellwiki/All_About_Monads

http://www.haskell.org/haskellwiki/Learn_Haskell_in_10_minut...

http://www.haskell.org/haskellwiki/Typeclassopedia

Also check out http://adit.io/ for some great tutorials with pictures

[codygman]

The functors, applicatives, and monads in pictures tutorial[1] just made so many things click for me. Those are all very very good resources indeed!

1. http://adit.io/posts/2013-04-17-functors,_applicatives,_and_...