Mach is a microkernel designed by Carnegie Mellon University from 1985 to 1994. Mach was one of the most popular (and worst) microkernels ever used. Compaq, IBM, HP all of them used the Mach microkernel at one time or another. GNU/Hurd was also based on Mach, but are undergoing the effort of switching to L4.
TakeDown.NET -> “Mach”