I build a micro Unix-like operating system called Jos following labs offered by graduate course 6.828 Operating System Engineering of MIT. The major parts of the JOS operating system are: Booting, Memory management, User-level environments, Preemptive multitasking, File system and spawn, a Shell and a NIC driver. The project is implemented using C in Linux, and uses Qemu to simulate virtual machine.
If you are interested in OS, I strongly recommeen you fellow lecutres and experiments offer by graduate course 6.828 Operating System Engineering of MIT (http://pdos.csail.mit.edu/6.828/2011/overview.html). I did the work at early 2012, so there may be a new version. The experiments is somehow hard that without help of other you may need a whole day or more to find right route or bug. I give my solution as reference.