The Haskell Platform, latest release. This package includes everything you need to get started, including a recent, stable version of GHC, standard libraries, and tools such as cabal (library management) and haddock (documentation creation).
Some way to edit Haskell source files. I use emacs and (an old version of) haskell-mode. I cannot recommend the latest version of haskell-mode, but come by my office hours if you would like help. A tutorial on setting up emacs for Haskell is here. There are many other IDEs for Haskell, and you are free to use whichever one you like.
Version control. You will need to work on your homework assignments collaboratively and some sort of version control that supports private repositories is essential. We recommend git and have prepared installation instructions to help you get started.
Haskell.org. Top-level page for Haskell open-source community.
Looking for a function but don't know what it's called? Want to see the documentation for a particular function? Hoogle searches many standard libraries and can search either by name or by type.
Hayoo is another search engine for the Haskell documentation, which is much more complete (it searches all of Hackage).
Real World Haskell, by Bryan O'Sullivan, Don Stewart, and John Goerzen, published by O'Reilly. A thorough and detailed introduction to Haskell that gets into the nitty gritty of using Haskell effectively in the "real world". Can be read online for free, or in dead tree form.
Learn You a Haskell for Great Good! is a whimsical and easy-to-follow Haskell tutorial, with super awesome illustrations.
School of Haskell provides tutorials for beginners through advanced Haskell programmers.
Parallel and Concurrent Programming in Haskell describes how functional programming matters in the context of multithreaded and multicore architectures.
The Haskell wikibook actually contains a substantial amount of well-written information; a great resource if you're having trouble understanding a particular topic and want a different approach.
The Haskell wiki is a huge grab-bag of all sorts of information, examples, explanations. The quality varies but it's definitely a great resource.
Brent Yorgey's famous Typeclassopedia explains many of the type classes in the standard libraries (Functor, Applicative, Monad, Monoid, Arrow, Foldable, Traversable...).
Planet Haskell aggregates blog posts from the Haskell community.
There is a Haskell subreddit for aggregating Haskell-related websites, blog posts, and news.
If you really want the nitty-gritty details of the Haskell language standard, see the 2010 Haskell report.
Martin Odersky's Keynote talk from Scala 2013 (Scala with style). This talk discusses good style in the Scala language, but much of what he says is valid in any language.
John Carmack's 2013 QuakeCon keynote address about his experiences with functional programming (interesting part from 2 mins in).
tryhaskell.org gives you a ghci session in your browser, and includes a very simple tutorial. It also features an interface to the #haskell IRC channel.
The #haskell IRC channel is a great place to get help. Strange as it may seem if you've spent time in other IRC channels, #haskell is always full of friendly, helpful people.
hpaste.org is a great place to paste programs you're having trouble with in order to get help from people in #haskell.
The Haskell-beginners mailing list is a good place to ask beginner-level questions.
The Haskell-cafe mailing list can also be a good place to ask questions, but is much higher-traffic.