[Haskell-cafe] As reading, what are your favorite Haskell libraries?

A wider question controls also for a library's applicability and (relative) absence of prerequisites, because the funnest reading is not only intrinsically fun, but also extrinsically rewarding and soon feasible (feasible to comprehend or to use). That is, I like simple, beautiful, powerful libraries that do not require me to first learn too many other things. I might value simplicity over power. I'm interested in answers to either question.