Which is your favorite "hidden gem" package on Hackage?
There are a lot of packages on Hackage, some well known (such as HUnit) and some less known (such as AspectAG). I'm wondering which package _you_ think is a hidden gem that deserves more users. Maybe a useful data structure, helpers for monads, networking, test, ...?

Which is your favorite "hidden gem" package on Hackage?
Which is your favorite "hidden gem" package on Hackage?
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There are a lot of packages on Hackage, some well known (such as HUnit) and some less known (such as AspectAG). I'm wondering which package _you_ think is a hidden gem that deserves more users. Maybe a useful data structure, helpers for monads, networking, test, ...?

Which is your favorite "hidden gem" package on Hackage?