Minix is back
When I was a Computer Science student I've learned some of the Operating System basics with Andy Tanembaum's Minix educational Unix-clone. It was not powerful but it worked on a PC with less than 1 MB of RAM off a floppy disk.
While the aim of Minix, and later on that of Minix 2, was to provide an educational tool, the new version Minix 3 is intended as a microkernel OS for embedded and resource-limitted computers and for applications that require high-reliability too (I like this latter one).
Minix 3 is still a work in progress and ports to other targets different than x86 are underway. But if you want to give a try you can download the CD or USB dongle image so you boot a live system and to perform a hard drive install. While not even close to what you get with a current GNU/Linux or FreeBSD it is still worth trying it out. Full C source is available.