OS Dev

>http://www.brokenthorn.com/Resources/OSDevIndex.html
Step by step from boot loader to protected mode, easily the best tutorial series for os dev

>http://www.cs.bham.ac.uk/~exr/lectures/opsys/10_11/lectures/os-dev.pdf
Great book on OS dev, including very interesting stuff about how c compiles into asm and how function work on the processor level.

>http://www.websofia.com/2011/10/writing-your-own-boot-loader-for-a-toy-operating-system-1/
Very easy to follow intro to writing a bootloader

>http://www.osdever.net/tutorials/
Various x86 OS tutorials

>http://www.ctyme.com/intr/int.htm
Interrupt jump table reference, has all bios functions documented

>http://www.bioscentral.com/misc/cmosmap.htm
Memory map for x86 boot, useful to know what's stored where

>http://www.nondot.org/sabre/os/articles/ProtectedMode/
Tutorials just for protected mode programming (no real mode stuff)

>http://www.cl.cam.ac.uk/projects/raspberrypi/tutorials/os/
>http://www.valvers.com/embedded-linux/raspberry-pi/step01-bare-metal-programming-in-cpt1
Raspberry PI Bare metal tutorial

>http://hackipedia.org/Platform/Nintendo/NES/html,%20NES%20ASM%20tutorials/nesasm.htm
Asm programming tutorial for NES

And some unsorted but useful links...

>http://www.supernovah.com/Tutorials/index.php
>http://www.codeproject.com/Articles/36907/How-to-develop-your-own-Boot-Loader#_Toc231383169
>http://geezer.osdevbrasil.net/osd/index.htm
>http://www.acm.uiuc.edu/sigops/roll_your_own/
>https://www.cs.uaf.edu/2005/fall/cs301/support/x86/nasm.html

Well, here are all the teaching OS concepts I have found on Wikipedia :
http://en.wikipedia.org/wiki/MINIX (inspired Linux)
http://en.wikipedia.org/wiki/Not_Another_Completely_Heuristic_Operating_System (or Nachos : I personnaly used it in school, pretty cool, it's an operating system with holes in it that you have to write yourself)
http://en.wikipedia.org/wiki/Pintos (looks like Nachos)
http://en.wikipedia.org/wiki/Tinix (apparently very tiny)
http://en.wikipedia.org/wiki/Xinu (the oldest one. "For the unrelated alien warlord, see Xenu.")
http://en.wikipedia.org/wiki/Xv6 (the MIT one)
https://github.com/tux3/SepplesOS