The concept of a microkernel dates to the early 1980's, it was observed that
the Unix kernel
s had grown in size and complexity to the point where the
ease of porting
to new architectures was becoming more difficult.
Perhaps a couple dozen or more base microkernels are listed at
http://www.cs.arizona.edu/people/bridges/os/microkernel.html. The best known of these include BeOS, Chorus, Mach, and NextSTEP, QNX and Hurd
(Mach based). Windows NT reportedly uses a microkernel.
In practice microkernels have been used to deliver a variety of specialized
benefits, including real time operation, and to deliver different operating
systems on a common hardware platform. The best known example of this is
IBM's use of a Mach-derived microkernel to deliver both AIX and as the
migration platform for os/400 when they discontinued the proprietary
as/400 architecture with power/powerpc.
BSD begat QNX and (arguably) Mach
Mach (bsd-flavored) begat: NextStep, AIX, osf/1, Hurd, MachTen and
Mac OS X
NextStep begat OpenSTEP which begat Rhapsody which begat Mac OS X
os/2 (v2.x) and VMS begat NT which begat Windows 2000
os/2 (v2.x) and Mach begat os/2 warp
osf/1 begat (DEC) osf/1 which begat Digital Unix which begat Tru64
AIX (as a microkernel) and os/400 begat os/400
This is a highly simplified view, and is kernel centric, and both differs
from Unix Family Tree and misses the inheritance of features. Most
authors statement of the taxonomy of unix is variant, release or
feature centered. That is part of the point of a microkernel, OS flavor
in a microkernel design is not coupled to the kernel design.
Note: Corrections, enhancements welcome. Operating Systems are very complex
in how they inherit code and features. (for instance I have no idea whether
plan 9 is implemented using a microkenel.)