Propellor is my second big Haskell program. I recently described the motivation for it like this, in a proposal for a Linux.Conf.Au talk:
The configuration of Linux hosts has become increasingly declarative, managed by tools like puppet and ansible, or by the composition of containers. But if a server is a collection of declarative properties, how do you make sure that changes to that configuration make sense? You can test them, but eventually it's 3 AM and you have an emergency fix that needs to go live immediately.
Data types to the rescue! While data types are usually used to prevent eg, combining an Int and a Bool, they can be used at a much more abstract level, for example to prevent combining a property that needs a Debian system with a property that needs a Red Hat system.
Propellor leverages Haskell's type system to prove the consistency of the properties it will apply to a host.
The real origin story though, is that I wanted to finally start using configuration management, but the tools for it all seemed very complicated and built on shaky foundations (like piles of yaml), and it seemed it would be easier to write my own than deal with that. Meanwhile, I had Haskell burning a hole in my pocket, ready to be used in a second large project after git-annex.
Propellor has averaged around 2.5 contributions per month from users since it got started, but increasing numbers recently. That's despite having many fewer users than git-annex, which remember gets perhaps 1 patch per month.
Of course, I've "cheated" by making sure that propellor's users know Haskell, or are willing to learn some. And, propellor is very compositional; adding a new property to it is not likely to be complicated by any of the existing code. So it's easy to extend, if you're able to use it.
At this point propellor has a small community of regular contributors, and I spend some pleasant weekend afternoons reviewing and merging their work.
Much of my best work on propellor has involved keeping the behavior of the program the same while making its types better, to prevent mistakes. Propellor's core data types have evolved much more than in any program I worked on before. That's exciting!