Haskell Learning Resources and References
This list, organized roughly by topic or theme, contains resources I have found to be most useful during my journey as a student of Haskell. It's not meant to be comprehensive, and I may have deliberately left some resources off of this list--there is more out there than I'm going to include here. But I consider these learning resources and references to be either the best out there, or otherwise foundational and important. (For example, I think the Haskell wiki sucks in a lot of ways, but there is enough important stuff on there that I would be remiss if I didn't include it in this list.)
Online Tutorials, Books, and Online Courses
This section contains resources that are meant to be gone through in a relatively linear fashion. I'll mark which ones I've gone through completely vs. which I've taken bits and pieces from.
Introduction to Functional Programming at edX, taught by Erik Meijer
I went through this entire course. This was the course that got me over the hump, so to speak, in Haskell. At the end of this course I had a relatively strong grasp of the basic algebraic structures--functors, monoids, monads, etc.--which are pervasive in Haskell, I felt pretty comfortable mentally parsing type signatures and understanding how to do basic stuff in Haskell. I really appreciate this course because there is an emphasis on fundamentals--like breaking down how an expression is evaluated in Haskell via redex analysis, and "following the types" to understand how to implement, say, a monad instance. Erik Meijer can be somewhat...eccentric and quite concise in his explanations, but if you bear with him, participate in forum discussions, and work your way through all of the exercises, you'll come out of the course with a solid foundation in Haskell.
CIS194 at University of Pennsylvania
I believe this was originally created by Brent Yorgey, but it looks like it's taught by others now. In any case, I have not gone through this course exhaustively, but pulled out exercises here and there to work on, and found it to be a great resource. Others who have gone through it more exhaustively suggest it's a great way for a self-learner to get a foundation in Haskell.
Graham Hutton's Programming in Haskell
I've only read parts of it when focusing specific topics, but that being the case, this is the best book I've found to use as a textbook for learning Haskell on your own. The sneaky way he teaches about monads via parsing without once using the 'm-word' blew me away the first time I encountered it (as an excerpt in Erik Meijer's course listed above, I should mention).
It's also worth checking out Graham Hutton's publications--lots of great stuff on folds in particular--an amazing resource.
Haskell on Wikibooks
A bit more uneven, but pretty decent in my experience. I've also only used this for focusing on specific topics but it's well-structured in that sense: the individual topic sections tend to be cohesive enough that you can pick and choose what you want based on where you're at in your learning process. Keep this one in your back pocket for when you need to focus on stuff like understanding better how the state monad works, or monoids...etc.
LYAH and Real World Haskell
Two resources, both with flaws, but still useful for beginners. If you try some of these other sites and don't like them, take a look at both of these and see if the style works for you. Note that Real World Haskell is kind of old at this point, so the specific approaches they take for things like parsing and whatnot may not be up-to-date with current best practices/libraries, but the foundation is still very solid.
School of Haskell
There's overlap here with some of the resources above, but it can be worth picking through this site especially to find topic-specific tutorials by pretty prominent members of the Haskell community. Some of the material here is quite advanced however.
Yesod Web Framework
I've actually not ever used the Yesod web framework, but I've found some of the sections of this online book incredibly useful for understanding modern Haskell development practices, especially for stuff like advanced Monad Transformer stack design and usage. Stuff like this can really put the more abstract, general documentation in context.
(Meta-)References
What I Wish I Knew When Learning Haskell
One of my favorite references for Haskell, especially useful as an "advanced beginner" or intermediate Haskell programmer. Worth checking out his section on monads before approaching this topic for some guidance on how to think about this contentious, overblown topic. Also see his summaries of various GHC extensions--useful for getting a basic grasp on the "why" of extensions that you've seen a lot but not yet used.
wiki.haskell.org
Kind of a mess, but with nuggets of gold throughout. Notable sections include the Typeclassopedia (by Brent Yorgey, mentioned above) which goes through some of the most important algebraic structures used in day-to-day Haskell; the section on folds (see the links at the bottom too, especially Foldr Foldl Foldl'), and more.
Haskell Hierarchical Libraries
Library documentation for stuff that is foundational or included by default in GHC. Probably a good place to start if you're asking questions like, "what is the Haskell lib for <standard data structure or set of functions you can expect to see in most programming languages>, and what module/package is it in?"
Glasgow Haskell Compiler User's Guide
Docs for the compiler you're probably using to compile Haskell. This is a great resource for libraries, GHCi details, profiling and debugging, and extensions too, in particular.
Stack docs
Docs for the system that you're probably using to install and create Haskell projects.
Hoogle
Great for looking up functions or type signatures to figure out where a type signature is coming from, what functions implement it, etc. Not sure why it says "type search doesn't work," seems like it does...?
Hackage
You'll probably end up coming here when trying to find the documentation for a specific Haskell package, e.g. mtl or containers, for example.
Specific Topics
FILL ME IN!
Monads
Monad Transformers
Type-level programming and dependent types in Haskell
Academic and Advanced Resources
I've barely dipped my toes in the water with most of these but it becomes obvious quickly if you spend any time with any one of these collections that there is enough knowledge here to last one a lifetime. Enjoy, don't drown...
Simon Peyton Jones's page at Microsoft Research
I heard this guy did some Haskell stuff shrugs
Philip Wadler's home page
Great, important papers here, written in an entertaining style. Professor Wadler is one of the people who helped create Haskell and has contributed a lot to the language over the years. Check out his papers on monads, Propositions as Types, and more for a background in some of the ideas that helped make Haskell what it is today.
okmij.org (Oleg Kiselyov's site)
Quite frankly, an incredibly intimidating collection of resources. Oleg is notable for his writing on all kinds of topics, but in particular, in the Haskell realm, I think his papers on Extensible Effects have been influential. I've only really touched a few items here and there--in particular I found his page on tagless-final style to be a useful guide on the subject (in particular the lecture notes/tutorial at the top: Typed Tagless Final Interpreters: Lecture Notes).
Graham Hutton's publications
Already mentioned above. Lots of good stuff here, and I find Professor Hutton's writing to be very approachable.
Richard Bird's publications
Not a lot linked to there but check out his book on Functional Pearls if you want to really blow your mind. I think I made it through like one or two of the articles in that book (which is a collection of articles taken from a recurring column in the Journal of Functional Programming. More context to be found on Prof. Jeremy Gibbons site here).
The Monad Reader
Collections of articles on various esoteric and not-so-esoteric topics in Haskell. "The Monad.Reader is a electronic magazine about all things Haskell. It is less formal than journal, but somehow more enduring than a wiki-page."
The Comonad Reader
A blog with a few different authors, (with apologies to Dan Doel and Gershom Bazerman and any other co-authors I may be forgetting) notably Edward Kmett.
I occasionally look at this blog, realize how out of my depth I am, bookmark it for later and close the tab. Something to aspire to.