How to Write an Operating System
Some hobby OSes:http://f2.fasm4u.net/hyos.htmlhttp://softwarewizard.dk/mojohttp://mikeos.berlios.dehttp://www.ctpp.co.uk/chaoshttp://www.returninfinity.com/baremetal.htmlhttp://mythago.net/enthflux.htmlA kernel development tutorial:http://www.jamesmolloy.co.uk/tutorial_html/index.html