The GNU Hurd is a set of servers running on top of the microkernel GNUMach. Eventually they will be ported to the L4 microkernel. This is the missing piece of the GNU Operating System that is currently provided by Linux, creating GNU/Linux.
The Hurd uses a microkernel that only controls the hardware and implements things like file systems and network protocols using user space programs called translators. The microkernel, when combined with the user space servers, provides all of the services of a standard monolithic kernel in a more modular fashion (at least in theory). Eventually even hardware device drivers will be implemented in user space. In theory, this is supposed to make writing and debugging things much easier as well as making the system much more scalable.