Haiku is an open-source operating system currently in development designed from the ground up for desktop computing.
Clicker is a project for a Multitasking Open-source operating system written from scratch. based on a modular micro kernel. We want to explore new way of designing and using OSes :) [newz: PCI, NE2K and VBE]
KoinKoin is an operating system, based on a second generation microkernel, k2 (strongly inspired by the L4 API). KoinKoin offers a modular design allowing to build specific-purpose and minimalist operating systems.
PenCube, the ONLY kernel for otakus.