Minix is an open source operating system that's been around for quite some time. I first tried it out sometime in the early 90's after downloading over some very slow dialup connection connected to a BBS.
From the post:
Andrew Tanenbaum said that it is important for a language like OCaml or Haskell to get more visibility among the OS developers; and Minix offers a very good way to do that. Minix3 is based on micro-kernel; all of the OS services (memory manager, file system, all the drivers, etc.) run as regular processes communicating through a well-defined protocol. It is irreleveant what language these services are written in, so long as they obey the protocol.
The poster goes on to suggest a way of slowly migrating the system routines to Haskell:
With Minix, we can replace one kernel service of the full-fledged, working OS with the one written in Haskell -- and immediately see how it all works. If it doesn't, the rest of OS still works and so we can unload the faulty service and load another implementation. Minix3 does indeed offer an easy road to the OS kernel for a language other than C. I said that there may be quite a bit of interest in Haskell community in such a project. I wonder if I'm right...