"The GNU Hurd is under active development. Because of that, there is no stable version." -- http://www.gnu.org/software/hurd/hurd.html

The Hurd kernel is used today. I know! When I worked for the NSA, all of our Cray supercomputers ran it. It can divide by zero and that is how we cracked passwords, AES 256, etc. It's a wonderful, albeit hippie-liberal kernel.

hurd is the operating system for duke nukem forever, didn't know that? ;-)