While CMU's OS class will always have a certain place in my heart, MIT's OS class has what is, in essence, a sample solution to the exercise of "write a little OS that basically kinda works" that is completely adorable. I gotta confess I haven't looked hard at a lot of such sample little OSes meant for teaching purposes (e.g., Minix, the Lions book, etc.) so maybe they're all kind of adorable like this. Still, this little fellow weighs in at less than 9kloc including a bunch of little userspace programs. Plus I have to give them a lot of credit for packaging it up nicely. How do you compile and run it? With a bunch of crazy autoconf magic? With ./configure make all make install qemu some-file ? No. Just "make qemu". It's all stupidly, beautifully dirt-simple C code that needs nothing to be configured. It compiles it, and then it runs qemu on the right thing.