What are the lesser-known but useful features of the Haskell programming language. (I understand the language itself is lesser-known, but work with me. Even explanations of the simple things in Haskell, like defining the Fibonacci sequence with one line of code, will get upvoted by me.) 

* Try to limit answers to the Haskell core
* One feature per answer
* Give an example and short description of the feature, not just a link to documentation
* Label the feature using bold title as the first line