QNX Neutrino RTOS (also "QNX6") is a RealTimeOperatingSystem which looks a lot like UNIX, and runs on PC hardware. It has a really nice GraphicalUserInterface (Photon), and will also run most UNIX software, like X11 and such. For a while, they were distributing a full-blown OS+GUI+MPEG Player demo version that fit on a single bootable floppy!
http://www.qnx.com, if you want to take a looksee. http://www.openqnx.com if you want to join the QNX community.
See MicroKernel.