Some hobby OSes:

http://f2.fasm4u.net/hyos.html

http://softwarewizard.dk/mojo

http://mikeos.berlios.de

http://www.ctpp.co.uk/chaos

http://www.returninfinity.com/baremetal.html

http://mythago.net/enthflux.html

A kernel development tutorial:

http://www.jamesmolloy.co.uk/tutorial_html/index.html