There's the GNU Hurd kernel.
I don't know if they ever got got something that would compile and work, though.
Edit:
http://en.wikipedia.org/wiki/Hurd
Here's a quote from the article "The latest release of the Hurd is version 0.2. It is fairly stable, suitable for use in non-critical applications. It is not yet suitable for a production environment."
So, apparently they did get it to work.