shell-monad is a small project, done over a couple days and not needing many changes since, but I'm covering it separately because it was a bit of a milestone for me.
As I learned Haskell, I noticed that the libraries were excellent and did things to guide their users that libraries in other languages don't do. Starting with using types and EDSLs and carefully constrained interfaces, but going well beyond that, as far as applying category theory. Using these libraries push you toward good solutions.
shell-monad was a first attempt at building such a library. The shell script it generates should always be syntactically valid, and never forgets to quote a shell variable. That's only the basics. It goes further by making it impossible to typo the name of a shell variable or shell function. And it uses phantom types so that the Haskell type checker can check the types of shell variables and functions match up.
So I think shell-monad is pretty neat, and I certianly learned a lot about writing Haskell libraries making it. Including how much I still have to learn!
I have not used shell-monad much, but keep meaning to make propellor and git-annex use it for some of their shell script needs. And ponder porting etckeeper to generate its shell scripts using it.