The Linux kernel (et al) hacked cleanly to run as a server on a Mach or other microkernel.
Binaries and information for PPC from:
http://www.mklinux.apple.com (tons of binaries, DRs, and documentation) Makes me wish I'd bought a PowerMac even more!
and for x86 from:
http://www.osf.org/mall/os/x86snap.htm (A snapshot from 26 Mar 96 and more recent binaries, a little information)