USB specifications

The official source of USB information. Not really easy to grasp, but mandatory if you want to know all about USB.

USB in a Nutshell (making sense of the USB standard)

A quote from the site:

"Starting out new with USB can be quite daunting. With the USB 2.0 specification at 650 pages one could easily be put off just by the sheer size of the standard. [...] The good news is you don't even need to bother reading the entire USB standard [...] Lets take a little journey through the various chapters of the USB 2.0 specification and briefly introduce the key points."

ELF - Executable and Linking Format - Specification v1.2 - pdf version

The official source of information about the ELF format. Apart from the specifications common to all platforms where the ELF standard is adopted, it also contains parts specific to the Intel architecture.

The Operating System Resource Center

A comprehensive set of links to various resources related to Operating Systems development, like file systems, device drivers, memory management, and so on.

Protected Mode Basics

An in-depth article by Robert Collins about protected mode on the x86 architecture. Probably contains more information about said architecture than you want to know... ;-)

Write your own operating system

Lots of useful information about the low-level bits of writing an operating system for the x86 architecture. Mostly about controlling the hardware, but also contains some useful titbits about file systems, networking, executable formats and other things...

Write your own operating system [WIKI]

Even more information about writing operating systems, although also geared towards the x86 architecture. Much information about the hardware, but also quite a lot of details on higher level things and some good references to useful books, tools and sources.