Xok
Xok (developed by MIT LCS Parallel and Distributed OSes group) is an OS based upon the idea of an "exokernel" (much like the no-kernel idea behind Tunes: there is no more runtime kernel in the OS, which yields up to 10000% performance gain).Also see Dawson Engler's page (now at Stanford), and the various Exokernel and networking papers.
This page is linked from: Aegis Microkernel Microkernel Debate