another thaught other than the GNU Mach kernel code would be to investigate the BSD series of kernels. They are built upon a system VERY similar to (same?) as Mach. I believe everyone is familiar with the BSD microkernel and the Lites server series. I personally believe that NetBSD and FreeBSD would be your best bet as an alternative to the GNU code. Also, there is the Darwin project (a derivative of FreeBSD... 4.4 or 4.5 i think) that has an ix86 port that you may want to investigate.
I personally prefer the GNU code because it already compiles on my linux box (w/ minimal work and adjustment) and there is an active CVS server with many patches and fixes on a regular basis (to my knowledge, at least).
I'm not exactly sure what your needs are, but I believe that this should help.
resources:
www.freebsd.org www.netbsd.org http://developer.apple.com/darwin/ http://www.gnu.org/software/hurd/download.html
>From: "Gregg C Levine"