There are a million monad tutorials for Haskell (and, with respect to their use in Haskell, it's probably better to learn them from there). I think that a good one is "You Could Have Invented Monads!".
This[1] is a good paper about monads in functional programming. It reads quite similarly to a monad tutorial, but with a little more formality.
As for actual category theory, I've enjoyed the first few chapters of "Algebra: Chapter 0", by Aluffi.
If you're after more type theory, the usual recommendations are "Types and Programming Languages", by Pierce, and "Software Foundations", also by Pierce.
I'd suggest http://eed3si9n.com/herding-cats/ . Though a lot of the time I find it's best to wait until you have a concrete use case, otherwise the concept is too abstract to make sense.
If you're looking for textbooks, Abstract and Concrete Categories—The Joy of Cats[0] by Adámek, Herrlich, and Strecker provides a nice introduction and is freely available as a PDF download.
I'm new here :)
Each time I see something like that, I think it looks great, but I'm having a hard time understanding the meaning of the vocabulary used…
Is there any good (free?) resources out there to learn Monoids, Monads, traits, category theory, in short, anything used in functional languages?
This is something that clearly stops me from using functional languages more.