The other answers are obsolete I think, today it is widely accepted I think that the best JSON library and the de-facto standard in Haskell is Aeson:
http://hackage.haskell.org/package/aeson
The maintainer is Brian O’Sullivan, known for his deep knowledge of Haskell and his performance-oriented work; there is no question it’s the right choice.
In addition Aeson offers a very nice API, check this example.
Also, I don’t know for the past but nowadays also Yesod uses Aeson:
http://hackage.haskell.org/packages/archive/yesod-json/1.1.0/doc/html/Yesod-Json.html
“using the aeson package”
http://www.yesodweb.com/book/json-web-service
“We’ll be using aeson for JSON parsing and rendering”