Writing a kernel is fun... I started writing one a few weeks ago. It's not a real one, just something to play with. But it's very interesting, probably because I'd never worked at such a low level before.
OSDev and OSDever ( http://osdever.net/ ) are both great resources in general, but I can't help but mention #osdev on Freenode. Incredibly useful resource if you're in need of assistance.
Great resource: http://www.osdev.org/