L4 (website) is a fast, new, sexy MicroKernel that is missing some functionality needed by TheHurd.
See :
Debian ITP 256506 pistachio -- L4 microkernel implementation http://lists.debian.org/debian-devel/2004/06/msg01425.html
http://www.cs.utah.edu/flux/oskit/ - oskit may also be of interest: